Minor fix in db_impl for compaction

Summary:

Test Plan:

Reviewers:

Subscribers:
Fixed a compile error of cuckoo_table_builder.

Summary:
Fixed the following compile error.

./table/cuckoo_table_builder.h:72:22: error: private field 'key_length_' is not used [-Werror,-Wunused-private-field]
  const unsigned int key_length_;
                     ^
1 error generated.

Test Plan: make

Reviewers: sdong, ljin, radheshyamb, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20349
Merge branch 'master' of github.com:facebook/rocksdb into rocks-compaction
[Compaction] Minor improvement on compaction code path

Summary:
This change will be applied in compaction branch.

Add more comments in compaction code path, consider multiple instead of
two-fixed level in MakeInputIterator

Test Plan:
make db_test -j32
export ROCKSDB_TESTS=Compaction
./db_test

Reviewers: sdong, ljin, igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20367