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%