correct logic related to Options::mmap_size and Options::limited_developer_mem.  was forcing all write_buffers to 20Mbyte.
correct logic related to Options::mmap_size and Options::limited_developer_mem.  was forcing all write_buffers to 20Mbyte.

basho/leveldb,icholy/leveldb,icholy/leveldb,tempbottle/leveldb-1,tempbottle/leveldb-1,icholy/leveldb,basho/leveldb,tempbottle/leveldb-1,basho/leveldb