Introduce progress output for fast cluster to cluster transfer (bb#709)

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 generic
transfers too. This is done if track_xfer parameter is set to positive
though.

Update sxcp tool to fit both direction transfers.
respect debug mode when going verbose (bb#728)
cache api rework
return file revisions in listfiles, do not return random crap for dirs
check revision presence in tests
make file lists cachable