glusterd: Add last successful glusterd lock backtrace

Also, moved the backtrace fetching logic to a separate function.
Modified the backtrace fetching logic able to work under memory pressure
conditions.

Change-Id: Ie38bea425a085770f41831314aeda95595177ece
BUG: 1138503
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/8584
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>