journal: Fixes reader that incorrectly returns io.EOF when last block is missing

Issue #61.
journal: Add more testing
leveldb: Introduce concurrent read benchmark
table: Recycle block buffer to reduce heap allocations #62

$ benchcmp old.txt new.txt
benchmark                       old ns/op     new ns/op     delta
BenchmarkDBRead                 494           432           -12.55%
BenchmarkDBReadConcurrent       135           121           -10.37%
BenchmarkDBReadConcurrent2      336           290           -13.69%
BenchmarkDBReadGC               658           420           -36.17%
BenchmarkDBReadUncompressed     417           365           -12.47%
BenchmarkDBReadTable            488           431           -11.68%
BenchmarkDBReadReverse          569           526           -7.56%
BenchmarkDBReadReverseTable     594           506           -14.81%
BenchmarkDBGet                  4957          5124          +3.37%
BenchmarkDBGetRandom            17436         11537         -33.83%

benchmark                       old MB/s     new MB/s     speedup
BenchmarkDBRead                 234.65       268.00       1.14x
BenchmarkDBReadConcurrent       855.75       955.83       1.12x
BenchmarkDBReadConcurrent2      344.66       399.74       1.16x
BenchmarkDBReadGC               176.20       275.82       1.57x
BenchmarkDBReadUncompressed     277.94       317.06       1.14x
BenchmarkDBReadTable            237.64       269.07       1.13x
BenchmarkDBReadReverse          203.52       220.22       1.08x
BenchmarkDBReadReverseTable     195.13       228.87       1.17x

benchmark                       old allocs     new allocs     delta
BenchmarkDBRead                 0              0              +0.00%
BenchmarkDBReadConcurrent       0              0              +0.00%
BenchmarkDBReadConcurrent2      0              0              +0.00%
BenchmarkDBReadGC               0              0              +0.00%
BenchmarkDBReadUncompressed     0              0              +0.00%
BenchmarkDBReadTable            0              0              +0.00%
BenchmarkDBReadReverse          0              0              +0.00%
BenchmarkDBReadReverseTable     0              0              +0.00%
BenchmarkDBGet                  16             17             +6.25%
BenchmarkDBGetRandom            20             22             +10.00%

benchmark                       old bytes     new bytes     delta
BenchmarkDBRead                 205           27            -86.83%
BenchmarkDBReadConcurrent       40            12            -70.00%
BenchmarkDBReadConcurrent2      193           43            -77.72%
BenchmarkDBReadGC               202           27            -86.63%
BenchmarkDBReadUncompressed     145           22            -84.83%
BenchmarkDBReadTable            207           27            -86.96%
BenchmarkDBReadReverse          260           82            -68.46%
BenchmarkDBReadReverseTable     262           82            -68.70%
BenchmarkDBGet                  963           908           -5.71%
BenchmarkDBGetRandom            6697          1124          -83.22%
leveldb: Fix forgotten error check #63
Bring back support for go1.2

Closes #65.
levledb: Fix testing for go1.2
leveldb: Recycle memdb to reduce heap allocations #62

$ benchcmp old.txt new.txt
benchmark                             old ns/op     new ns/op     delta
BenchmarkDBWrite                      3468          3301          -4.82%
BenchmarkDBWriteBatch                 1100          1044          -5.09%
BenchmarkDBWriteUncompressed          3204          3182          -0.69%
BenchmarkDBWriteBatchUncompressed     1094          1016          -7.13%
BenchmarkDBWriteRandom                4208          3845          -8.63%
BenchmarkDBWriteRandomSync            3521          3349          -4.88%
BenchmarkDBOverwrite                  3455          3430          -0.72%
BenchmarkDBOverwriteRandom            3916          3856          -1.53%
BenchmarkDBPut                        3883          4057          +4.48%

benchmark                             old MB/s     new MB/s     speedup
BenchmarkDBWrite                      33.45        35.14        1.05x
BenchmarkDBWriteBatch                 105.44       111.04       1.05x
BenchmarkDBWriteUncompressed          36.20        36.44        1.01x
BenchmarkDBWriteBatchUncompressed     105.96       114.06       1.08x
BenchmarkDBWriteRandom                27.56        30.17        1.09x
BenchmarkDBWriteRandomSync            32.94        34.63        1.05x
BenchmarkDBOverwrite                  33.57        33.81        1.01x
BenchmarkDBOverwriteRandom            29.62        30.08        1.02x
BenchmarkDBPut                        29.87        28.59        0.96x

benchmark                             old allocs     new allocs     delta
BenchmarkDBWrite                      5              5              +0.00%
BenchmarkDBWriteBatch                 1              1              +0.00%
BenchmarkDBWriteUncompressed          5              5              +0.00%
BenchmarkDBWriteBatchUncompressed     1              1              +0.00%
BenchmarkDBWriteRandom                5              5              +0.00%
BenchmarkDBWriteRandomSync            5              5              +0.00%
BenchmarkDBOverwrite                  5              5              +0.00%
BenchmarkDBOverwriteRandom            5              5              +0.00%
BenchmarkDBPut                        7              7              +0.00%

benchmark                             old bytes     new bytes     delta
BenchmarkDBWrite                      452           201           -55.53%
BenchmarkDBWriteBatch                 386           102           -73.58%
BenchmarkDBWriteUncompressed          451           176           -60.98%
BenchmarkDBWriteBatchUncompressed     385           100           -74.03%
BenchmarkDBWriteRandom                464           228           -50.86%
BenchmarkDBWriteRandomSync            452           200           -55.75%
BenchmarkDBOverwrite                  492           174           -64.63%
BenchmarkDBOverwriteRandom            491           172           -64.97%
BenchmarkDBPut                        660           503           -23.79%
util: Fix build on go1.2
table: blockIter: Nil'ing the block
leveldb: Introduce 'leveldb.blockpool' property #62
cache: Simplify its API and (probably) some bugfix

Probably has something todo with #62 and #67.
util: Adds helper to create range form bytes prefix #66
leveldb: Export more DB properties
table: Fix imbalance bpool Get and Put; and use buffer capacity as cache charge #62
table: Includes index and filter block in the block cache #62
table: Implements releaser in the reader
cache: Add convenience APIs
util: BufferPool: Replace sync.Pool with channel #62
leveldb: Use channel for memdb pool #62