MB-7523: added master events support for backfill ended event
MB-7523: added master events support for backfill ended event

API will be used in next commit. We need ability to trace those
events.

Change-Id: I78a787de0c32d192b58232c22a0d59de8bc1d4f9
Reviewed-on: http://review.couchbase.org/24066
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

membase/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,vzasade/ns_server,daverigby/ns_server,ceejatec/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,mschoch/ns_server,membase/ns_server,membase/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,mschoch/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,daverigby/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,membase/ns_server