[HBASE-4027] Enable direct byte buffers LruBlockCache - ASF JIRA

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Setting hbase.offheapcachesize in hbase-site.xml and -XX:MaxDirectMemorySize in hbase-env.sh to enable this feature. The file already has a line you can uncomment and you need to set the size of the direct memory (your total memory - size allocated to memstores - size allocated to the normal block cache - and some head room for the other functionalities).

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Setting hbase.offheapcachesize in hbase-site.xml and -XX:MaxDirectMemorySize in hbase-env.sh to enable this feature. The file already has a line you can uncomment and you need to set the size of the direct memory (your total memory - size allocated to memstores - size allocated to the normal block cache - and some head room for the other functionalities).

Java offers the creation of direct byte buffers which are allocated outside of the heap.

They need to be manually free'd, which can be accomplished using an documented clean method.

The feature will be optional. After implementing, we can benchmark for differences in speed and garbage collection observances.

1.	Metrics panel for the Off Heap Cache	Closed	Unassigned
2.	Investigate different memory allocation models for off heap caching.	Closed	Unassigned
3.	Race condition in Slab.java that occurs due to spinlock unlocking early	Closed	Li Pi
4.	Move spinlock to SingleSizeCache rather than the slab allocator.	Closed	Li Pi
5.	race condition in CacheTestUtils	Closed	Li Pi
6.	SlabCache metrics bugfix.	Closed	Li Pi
7.	Off Heap Cache never creates Slabs	Closed	Li Pi