MB-10073: expose xdcr paused-ness in tasks API

I.e. so that UI and tools can observe if/when xdcr replication is
actually paused.

Change-Id: I22ed94a12c42d5a19724328cd5d7319078d3269e
Reviewed-on: http://review.couchbase.org/33710
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>