MB-7217: implemented hidden setting for faster xdcr checkpoints

If ns_config variable xdcr_commits_dont_wait_disk is set to true we
will not wait until replicated mutations are persisted.

New checkpointing protocol and upr failover log still allow us to
detect data loss if replicated data is not preserved. Default is still
to wait for persistence as part of checkpointing in order to make data
loss less probable. I.e. our limitation is that we only keep latest
checkpoint. So if our checkpoint becomes invalid due to failover/crash
we're forced to re-replicate from scratch. Waiting for disk on target
prevents this (arguably rare) condition.

Change-Id: Iaf58f3244656799746a141a9f405a5401ac8df42
Reviewed-on: http://review.couchbase.org/40483
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>