log vbucket mover's pids for better rebalance logging
log vbucket mover's pids for better rebalance logging

Currently we have quite verbose replication logging. But rebalance
ebucketmigrators are a bit less verbose. During recent bug
investigation lack of this information was impediment.

Change-Id: I1d7d3ccbf31b76568c01c0de8e4e2e890055588d
Reviewed-on: http://review.couchbase.org/7816
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>

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