Summary:
This diff adds BlockBasedTable format_version = 2. New format version brings better compressed block format for these compressions:
1) Zlib -- encode decompressed size in compressed block header
2) BZip2 -- encode decompressed size in compressed block header
3) LZ4 and LZ4HC -- instead of doing memcpy of size_t encode size as varint32. memcpy is very bad because the DB is not portable accross big/little endian machines or even platforms where size_t might be 8 or 4 bytes.

It does not affect format for snappy.

If you write a new database with format_version = 2, it will not be readable by RocksDB versions before 3.10. DB::Open() will return corruption in that case.

Test Plan:
Added a new test in db_test.
I will also run db_bench and verify VSIZE when block_cache == 1GB

Reviewers: yhchiang, rven, MarkCallaghan, dhruba, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D31461