gsyncd / geo-rep : failover/failback

This commit is based on Venky Shankar <vshankar@>'s
original implementation. Let us first quote Venky's
description, then we summarize changes to his work.

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First version of failover/failback.

Failback mechanism uses two exclusive modes:
  * blind-sync
    This mode works with xtime pairs (both master and slave) to
    identify candidated to sync the original master from the slave

  * wrapup-sync
    This mode is similar to the normal working of gsyncd except
    that orphaned entities in the gluster volume are not assigned
    xtimes. This prevents un-necessary transfer of data for such
    entities.

Modes can be enabled via:

  gluster volume geo-replication M S config special_sync_mode blind
  gluster volume geo-replication M S config special_sync_mode wrapup

To turn off the special modes (i.e. to revert to normal gsyncd behaviour) use:

  gluster volume geo-replication colon-d0 192.168.1.2::colon-d config \!special_sync_mode
------

Code has been refactored to meet following goals:

- make checkpointing work with special sync modes
- move out sync mode related conditionals from the crawl
  loop and make all decisions to be made at startup
  time
- be intrusive to the crawl loop to smallest possible degree
  (we will have to change/revisit it for other reasons,
  and the complexity of that should not increase)

So, xtime parsing/updating/evaluation that's specific to
the certain special modes are represented as mixin classes;
basic operation logic is in an abstract base class.
On startup, special-sync-mode tunable is dynamically dispatched
to the corresponding mixin and the actual master class is
derived from the chosen mixin and the ABS.

Change-Id: Ic9b8448f31ad4239a8200dc689f7d713662a67de
BUG: 830497
Signed-off-by: Csaba Henk <csaba@redhat.com>

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