[HBASE-8837] Initial commit of SLAB allocator for the MemStore.

Author: shaneh

Summary:
Slab allocator for HBase MemStore. Uses direct byte buffers
to manage a slab cache for the MemStore to address fragmentation
issue that causes stop the world garbage collections in the JVM.

Test Plan: Tested on HBASEDEV139 using YCSB to generate load.

Reviewers: liyintang, rshroff, manukranthk, adela

Reviewed By: manukranthk

CC: hbase-eng@, arice

Differential Revision: https://phabricator.fb.com/D835989

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1498167 13f79535-47bb-0310-9956-ffa450edef68