Adds limited support for the FETCH_FIRST fetch orientation in HS2 client requests.

Adds a bounded query-result cache that clients can enable by setting
an 'impala.resultset.cache.size'  option in the HS2 confOverlay mapof the HS2 exec request.
Impala permits FETCH_FIRST for a particular stmt iff result caching is enabled.
FETCH_FIRST will succeed as long all previously fetched rows fit into the bounded
result cache. Regardless of whether a FETCH_FIRST succeeds or not, clients may
always resume fetching with FETCH_NEXT.

The FETCH_FIRST feature is intended to allow HUE users to export an entire
result set (to Excel, CSV, etc.) after browsing through a few pages of results,
without having ro re-run the query from scratch.

Change-Id: I71ab4794ddef30842594c5e1f7bc94724d6ce89f
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1356
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins