Admission controller: protect queue access and mark pools for updates

This fixes two bugs:
1) AdmitQuery() accesses the request_queue_map_ without holding the
   lock and the unordered_map is not thread-safe.
2) Dequeued requests were not causing pool stats to be sent.

Change-Id: I6d21dd69a6c8b59da0506d90f97ad0d996d8751b
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1488
Reviewed-by: Henry Robinson <henry@cloudera.com>
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins