debug/trace: save the recent fops received in the event-history

* Make use of event-history in debug/trace xlator to dump the recent fops,
when statedump is given. trace xlator saves the fop it received along
with the time in the event-history and upon statedump signal, dumps its
history. The size of the event-history can be given as a xlator option.

* Make changes in trace to take logging into log-file or logging to
history as an option. By default both are off.

Change-Id: I12baee5805c6efb55735cead4e2093fb94d7a6a0
BUG: 797171
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4088
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>