Summary:
Provide a block_align option in BlockBasedTableOptions to allow
alignment of SST file data blocks. This will avoid higher
IOPS/throughput load due to < 4KB data blocks spanning 2 4KB pages.
When this option is set to true, the block alignment is set to lower of
block size and 4KB.
Closes https://github.com/facebook/rocksdb/pull/3502

Differential Revision: D7400897

Pulled By: anand1976

fbshipit-source-id: 04cc3bd144e88e3431a4f97604e63ad7a0f06d44