HashLinkedList::Iterator: remove an ununsed class variable

Summary: This variable is not used. Remove it.

Test Plan: build.

Reviewers: haobo, igor, yhchiang

Reviewed By: haobo

CC: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18525
Compaction with zero outputs

Summary: We had a hypothesis in https://reviews.facebook.net/D18507 that empty-string internal keys might have been caused by compaction filter deleting all the entries. I added a unit test for that case. Unforutnately, everything works as expected.

Test Plan: this is a test

Reviewers: dhruba, haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18519
autovector::resize

Summary: Resize the autovector!

Test Plan: test

Reviewers: sdong

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18543
MergingIterator uses autovector instead of vector

Summary:
Use autovector in MergingIterator so that if there are 4 or less child iterators in it, iterator wrappers are inline, which is more likely to be cache friendly.

Based on one test run with a shadow traffic of one product, it reduces CPU of MergingIterator::Seek() by half.

Test Plan: make all check

Reviewers: haobo, yhchiang, igor, dhruba

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18531
Make RocksDB work with newer gflags

Summary:
Newer gflags switched from `google` namespace to `gflags` namespace. See: https://github.com/facebook/rocksdb/issues/139 and https://github.com/facebook/rocksdb/issues/102

Unfortunately, they don't define any macro with their namespace, so we need to actually try to compile gflags with two different namespace to figure out which one is the correct one.

Test Plan: works in fbcode environemnt. I'll also try in ubutnu with newer gflags

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18537