HashLinkList memtable switches a bucket to a skip list to reduce performance outliers

Summary:
In this patch, we enhance HashLinkList memtable to reduce performance outliers when a bucket contains too many entries. We switch to skip list for this case to enable binary search.

Add threshold_use_skiplist parameter to determine when a bucket needs to switch to skip list.

The new data structure is documented in comments in the codes.

Test Plan:
make all check
set threshold_use_skiplist in several tests

Reviewers: yhchiang, haobo, ljin

Reviewed By: yhchiang, ljin

Subscribers: nkg-, xjin, dhruba, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D19299
Revert "Two small fixes in db_test"

This reverts commit 6634844dba962b9a150646382f4d6531d1f2440b.
Re-commit the correct part (WalDir) of the revision:

Commit 6634844dba962b9a150646382f4d6531d1f2440b by sdong
Two small fixes in db_test

Summary:
Two fixes:
(1) WalDir to pick a directory under TmpDir to allow two tests running in parallel without impacting each other
(2) kBlockBasedTableWithWholeKeyHashIndex is disabled by mistake (I assume). Enable it.

Test Plan: ./db_test

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: nkg-, igor, dhruba, haobo, leveldb

Differential Revision: https://reviews.facebook.net/D19389
Fix 32-bit errors

Summary: https://www.facebook.com/groups/rocksdb.dev/permalink/590438347721350/

Test Plan: compiles

Reviewers: sdong, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19197
Centralize compression decision to compaction picker

Summary:
Before this diff, we're deciding enable_compression in CompactionPicker and then we're deciding final compression type in DBImpl. This is kind of confusing.

After the diff, the final compression type will be decided in CompactionPicker.

The reason for this is that I want CompactFiles() to specify output compression type, so that people can mix and match compression styles in their compaction algorithms. This diff makes it much easier to do that.

Test Plan: make check

Reviewers: dhruba, haobo, sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19137
Support Multiple DB paths (without having an interface to expose to users)

Summary:
In this patch, we allow RocksDB to support multiple DB paths internally.
No user interface is supported yet so this patch is silent to users.

Test Plan: make all check

Reviewers: igor, haobo, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18921
Add TimedWait() API to CondVar.

Summary:
Add TimedWait() API to CondVar, which will be used in the future to
support TimedOut Write API and Rate limiter.

Test Plan: make db_test -j32

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19431