Correct default value for SetWriteBufferSize
Correct default value for SetWriteBufferSize

According to https://github.com/facebook/rocksdb/wiki/Setup-Options-and-Basic-Tuning#column-family-write-buffer-size default for `SetWriteBufferSize` should be `64MB` and not `4MB`
tecbot/gorocksdb,tecbot/gorocksdb