fix memory leak in two_level_iterator

fix memory leak in two_level_iterator

Summary:
this PR fixes a few failed contbuild:
1. ASAN memory leak in Block::NewIterator (table/block.cc:429). the proper destruction of first_level_iter_ and second_level_iter_ of two_level_iterator.cc is missing from the code after the refactoring in https://github.com/facebook/rocksdb/pull/3406
2. various unused param errors introduced by https://github.com/facebook/rocksdb/pull/3662
3. updated comment for `ForceReleaseCachedEntry` to emphasize the use of `force_erase` flag.
Closes https://github.com/facebook/rocksdb/pull/3718

Reviewed By: maysamyabandeh

Differential Revision: D7621192

Pulled By: miasantreble

fbshipit-source-id: 476c94264083a0730ded957c29de7807e4f5b146