IMPALA-749: Fetch partitions in batches, rather than all at once.

This updates how Impala fetches partition metadata from the Hive Metastore to fetch
partitions in batches, rather than all at once. This helps reduce the load on the
HMS and also lets Impala scale to above 32K partitions. The downside is that it
may require additional RPCs to get all the partitions.

This is done by first querying the metastore to get all the partition names that
exist, then splitting the list of names into seperate batches to get the actual
partition metadata.

Impala uses a default size of 1000 partitions per batch, but it can be configured
by setting the 'hive.metastore.batch.retrieve.table.partition.max' parameter
in the hive-site.xml config file.

Change-Id: Ide0ec30ef8a9e00f79c26551aa8e5e7814c73034
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1662
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins