Use ProcessClock instead of hrtime_t, which cannot really wrap (and hence can remove the extra checks)

See comments on http://review.couchbase.org/#/c/71572/3/src/vbucket.cc@967 - we unnecessarily check that hrtime_t could go backwards, which isn't possible if we use ProcessClock:

void VBucket::updateBGStats(const hrtime_t init,

                            const hrtime_t start,

                            const hrtime_t stop) {

    if (stop >= start && start >= init) {

    ....

Should replace uses of hrtime_t here with ProcessClock (when we are measuring the duration of something in a single thread).