Fix hash table buckets to allocate memory from the BlockMgr.

This was always a TODO. We want memory to come from the block mgr and trigger spilling.

Change-Id: I07f1f79fbbb33068fb2df64510a80a9b008ef73d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4466
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>