UDF/UDA memory management improvements

* AggFnEvaluator now uses the UDF mem pool (I'm planning to change
  this to per-exec node pools in the expr refactoring)
* FunctionContext::TrackAllocation()/Free() actually use the UDF's mem tracker
* Added FunctionContextImpl::Close() which sets warnings for leaked allocations

Change-Id: I792ffd49102a92b57e34df18d8ff5f5d0fd27370
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1792
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: Skye Wanderman-Milne <skye@cloudera.com>