xtimes are transferred from master to slave synchronously
Above op is a major bottleneck especisally for small file layouts.
Add a mode where rsync is used to transfer xtimes (and other extended attributes).
Partial fix -- increased speed, reduced correctness, cf. doc text of https://bugzilla.redhat.com/show_bug.cgi?id=764075.
Alternative approach would be to implement asynchronous xtime transfer.