[MB-11548] Memcached does not handle going back in time.

Add a new cb_time method that allow for a platform independent way of
obtaining a monotonic second time value.

Add a new cb_time method that abstracts gettimeofday (even if we do reimplement
gettimeofday for Win32).

The new cb_get_timeofday method has a related cb_set_timeofday_offset that enables easier
testing of time-shift bugs.

Change-Id: Ib24d628c5d58e7e9236c4635fff966875053b45d
Reviewed-on: http://review.couchbase.org/39811
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>