Add log message to track container requests in yarn resourcemanager

Log requests from AM to RM for containers.
Tracked in community as YARN-2806 (patch submitted).

The check, !lastRequest.equals(request), prevents the log from being spammed by repeated duplicate requests from the AM. Before I added this check the log size grew 10X because the AM submits duplicate requests that piggyback on heartbeat messages, even though the requests don't actually change.

Limiting the logged information to actual changes in resources requested causes the additionally log overhead to be negligible.