MB-7432: reimplemented xdcr stats

We now maintain global ETS table (xdcr_stats) which allows us to
efficiently accumulate xdcr stats without having to send them to
per-replication process.

Change-Id: I4171090d69d2fad978f647efb03e035049e51b95
Reviewed-on: http://review.couchbase.org/40094
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>