Introduce fast remote to remote progress output (bb#709)

Rework xfer_stat structure placement. Now it is placed in sxi_conns_t
structure not in sxc_cluster_t structure. This gives a possibility to
easily access it from other types of events than file uploads and
downloads.

Change sxi_cluster_query function to be a wrapper of
sxi_cluster_query_track. Now xfer_stat structure is passed to struct
generic_ctx context to enable progress tracking for normal generic
transfers too. This is done if track_xfer parameter is set to positive
though.

Update sxcp tool to fit both direction transfers.