IMPALA-887: Improve partition pruning time

This commit is the first step in improving the performance of partition
pruning. Currently, Impala can prune approximately 10K partitions per
sec, thereby introducing significant overhead for huge table with a
large number of partitions. With this commit we reduce that overhead by
3X by batching the partition pruning calls to the backend.

Change-Id: I3303bfc7fb6fe014790f58a5263adeea94d0fe7d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2608
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins