Add table reader GetIntProperties.
Backporting the new table reader properties from rocksdb 6.x. Also update write_buffer_manager.h with final code that was accepted by Facebook to rocksdb 6.x (Facebook found a minor flaw which is now fixed).
Backporting the new table reader properties from rocksdb 6.x. Also update write_buffer_manager.h with final code that was accepted by Facebook to rocksdb 6.x (Facebook found a minor flaw which is now fixed).