IMPALA-963: Impala crash in ClearResultCache()

The issue is that Impala crashes in ClearResultCache() with result caching on
for parallel inserts. The reason is that the ClearResltCache() accesses the
coordinator RuntimeState to update the query mem tracker. However, for there is
no coordinator fragment (or RuntimeState) for parallel inserts.
The fix is to intiialize a query mem tracker to track memory usage in the coordinator
instance even if there is no coordinator fragment.

Change-Id: I3a2ef14860f683910c29ae19b931202ca6867b9f
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2454
Reviewed-by: Srinath Shankar <sshankar@cloudera.com>
Tested-by: jenkins