synctask: add backtrace per waiting task

The backtrace is 'saved' in a per-task buffer.
This would come handy while debugging code using
synctasks.

Change-Id: I732b275f6d15b31f31361f5ecf2ba47cacde9b54
BUG: 1138503
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/8622
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>