Admission controller: add memory limits in addition to number of requests

Adds the ability to set per-pool memory limits. Each impalad tracks the memory
used by queries in each pool; a per-pool memory tracker is added between the
per-query trackers and the process memory tracker. The current memory usage
is disseminated via statestore heartbeats (along with the other per-pool stats)
and a cluster-wide estimate of the pool memory usage is updated when topic
updates are received. The admission controller will not admit incoming
requests if the total memory usage is already over the configured pool limit.

Change-Id: Ie9bc82d99643352ba77fb91b6c25b42938b1f745
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1508
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins