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
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