If 'autocompress' in s3 configure file is set to 'true', all data will be
compressed before uploaded to S3. In this way, we can reduce the network
traffic significantly, which means money-saving as well.

The data will be compressed in 'NO_FLUSH' way before injected into underlayer
S3KeyWriter's buffer, and the latter will invoke RESTFul layer to finish data
uploading. We don't buffer data issued from s3extprotocol in compression layer,
that is, all data blocks will be injected into ZStream to deflate immediately.
It is because experimental results show little performance improvement by doing
this while consuming more memory.

Signed-off-by: Peifeng Qiu, Adam Lee