add s3 an underlayer fs option.
init mUri for hadoop FS impl.
allow rawtable to update its metadata, with unit tests.
1. log metadata change for rawtable. 2. make getRawTableMetadata return a writable buffer.
UI works no matter where the cmd is exec.
1. masterlogwrite provide option to not flush. 2. preliminary version for client side metacache. 3. more tests.