Remove TQueryGlobals and introduce a TQueryContext.

This change simplifies the creation and maintenance of query-global
parameters required for evaluating special expressions such as now(),
user(), etc. that should return a consistent result regardless of
which impalad they are evaluated on. These parameters are now part of
a new TQueryContext which contains the original TClientRequest and the
TSessionState.

Change-Id: Ib36ccdd0640413dc5bf46ef261ca1aa4ec0adec5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1253
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins