Adding aliases ADD_MONTHS and SUB_MONTHS

This is a request for consistency with oracle.

Change-Id: I463a66694a068cd773532d8f6f853a4b089b918a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2400
Reviewed-by: Victor Bittorf <victor.bittorf@cloudera.com>
Tested-by: jenkins
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