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