Add option verify_checksums_in_compaction

Summary:
If verify_checksums_in_compaction is true, compaction will verify checksums. This is default.
If it's false, compaction doesn't verify checksums. This is useful for in-memory workloads.

Test Plan: corruption_test

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16695
[RocksDB] LogBuffer Cleanup

Summary: Moved LogBuffer class to an internal header. Removed some unneccesary indirection. Enabled log buffer for BackgroundCallFlush. Forced log buffer flush right after Unlock to improve time ordering of info log.

Test Plan: make check; db_bench compare LOG output

Reviewers: sdong

Reviewed By: sdong

CC: leveldb, igor

Differential Revision: https://reviews.facebook.net/D16707
Dump options in backupable DB

Summary: We should dump options in backupable DB log, just like with to with RocksDB. This will aid debugging.

Test Plan: checked the log

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16719
fix ldb_test TtlPutGet test

Summary:
If the last byte of the timestamp is 0, the output value of the ttl db
can cut off earlier. Change to compare hex format instead.

Test Plan:
this does not happen consistently, but it did not fail after the last
100 runs
for i in {1..100}; do python ./tools/ldb_test.py; done

Reviewers: igor, haobo

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16725
[RocksDB] make sure KSVObsolete does not get accessed as a valid pointer.

Summary: KSVObsolete is no longer nullptr and needs to be checked explicitly. Also did some minor code cleanup and added a stat counter to track superversion cleanups incurred in the foreground.

Test Plan: make check

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16701
Consolidate SliceTransform object ownership

Summary:
(1) Fix SanitizeOptions() to also check HashLinkList. The current
dynamic case just happens to work because the 2 classes have the same
layout.
(2) Do not delete SliceTransform object in HashSkipListFactory and
HashLinkListFactory destructor. Reason: SanitizeOptions() enforces
prefix_extractor and SliceTransform to be the same object when
Hash**Factory is used. This makes the behavior strange: when
Hash**Factory is used, prefix_extractor will be released by RocksDB. If
other memtable factory is used, prefix_extractor should be released by
user.

Test Plan: db_bench && make asan_check

Reviewers: haobo, igor, sdong

Reviewed By: igor

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D16587
[RocksDB] Minor cleanup of PurgeObsoleteFiles

Summary: as title. also made info log output of file deletion a bit more descriptive.

Test Plan: make check; db_bench and look at LOG output

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16731