Use MsgPack for xlog/snaps files and IPROTO for replication

 * Change format of xlog/snap files to MsgPack-based format
 * Update replication to use IPROTO protocol
 * Rework replica bootstrap to process snapshot by rows
 * Add support for composite rows in fio_batch
 * Fix iproto->sync in SUBSCRIBE command
Merge remote-tracking branch 'origin/master' into logrow-msgpack