libxlator: implement pluggable aggregation policies

The API is described in libxlator.h.

Behavior remains the same for this commit; this
is a preparatory step for per-translator customization
of aggregation.

Change-Id: I5d42923af59b2fd78e1ff59c12763875b57c5190
BUG: 847839
Original Author: Csaba Henk <csaba@redhat.com>
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4903
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>