Summary: Currently, if users didn't set options.arena_block_size, we set "result.arena_block_size = result.write_buffer_size / 10". It makes result.arena_block_size not a multiplier of 4KB, even if options.write_buffer_size is a multiplier of MBs. When calling malloc to arena_block_size, we may waste a small amount of memory for it. We now make the default to be /8 or /16 and align it to 4KB.
Test Plan: unit tests
Reviewers: sdong
Reviewed By: sdong
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D46467
b5b2b75e52