MB-10800: avoid overflow computing cb_cond_timedwait wakeup time

tv_sec is usually long. Which is usually 32-bit on 32-bit
machines. And multiplying it by 1000, even if final result is
uint64_t, is still performed as 32-bit computation on 32-bit boxes.

Clearly, multiplying current unix-epoch time by 1000 overflows 32-bit.

The fix is to simply convert tv_sec to uint64_t before multiplication
which ensures that it's computed using 64-bit arithmetics.

Change-Id: I13f6a6413bd3b452dfce98eb68fcb1f5daa7f58d
Reviewed-on: http://review.couchbase.org/35919
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>