MB-7523:ebucketmigrator:added ability to wait for end of backfill

We're spoofing backfill end message flying over tap stream to detect
when backfill is done. It'll be used in later commit to start
backfilling next vbucket as soon as backfilling of current vbucket is
complete.

Change-Id: I840b948c3420e0449a71f9a5f7b4e4f777ab79b5
Reviewed-on: http://review.couchbase.org/24065
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
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 <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>