Add client side snappy compression to cbbackupmgr

Utilize the datatype field in order to decide whether or not a document is compressed and compress it. We should support persisting all documents in compressed form, uncompressed form, or as is form which is just a mixture of compressed and uncompressed documents. The initial implementation should utilize the datatype field, but since this is not implemented in the server we will need to uncompress al documents before restoring them. Once Couchbase supports the datatype field we can send documents back in the various forms.