timer: don't reschedule timer bucket in a domain

If two timers run on the same tick, and the first timer uses a domain,
and then catches an exception and disposes of the domain, then the
second timer never runs. (And even if the first timer does not dispose
of the domain, the second timer could run under the wrong domain.)

This happens because timer.js uses "process.nextTick()" to schedule
continued processing of the timers for that tick. However, there was
an exception inside a domain, then "process.nextTick()" runs under
the domain of the first timer function, and will do nothing if
the domain has been disposed.

To avoid this, we temporarily save the value of "process.domain"
before calling nextTick so that it does not run inside any domain.