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
drop useless dead code
temporary workaround sxls to support old servers
Improve sxls efficiency by storing previous query result in memory and skipping unnecessary queries.

This improvement let database to take advantage of indexing and reduces
redundant queries. More text processing is done inside sqlite engine
than it was done before.