Perform prioritized load requests for missing tables in HS2 metadata ops

The HS2 metadata operations do not go through analysis() so the prioritized
loading will not happen for them. Most of the HS2 metadata ops work purely
on table/db names, but GetColumns() requires loading the table metadata. This
patch updates MetadataOp to collect a set of missing tables and request these
tables be loaded from the catalog server. The operation will wait until the tables
are loaded in the local catalog before proceeding.

Change-Id: I070f2a0d9194d3317f09431971be9a8dffbc7386
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1542
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins