IMPALA-786: Drop function should clear library cache.

We were previously only clearing the cache in the catalog service
update loop so the impalad the drop was issued to was not doing the
right thing.

Change-Id: I6bee228e8c0d565cea4ea61cbf64240d83a45a7d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1445
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins