timers: Avoid linear scan in _unrefActive.

Before this change, _unrefActive would keep the unrefList sorted when
adding a new timer.

Because _unrefActive is called extremely frequently, this linear scan
(O(n) at worse) would make _unrefActive show high in the list of
contributors when profiling CPU usage.

This commit changes _unrefActive so that it doesn't try to keep the
unrefList sorted. The insertion thus happens in constant time.

However, when a timer expires, unrefTimeout has to go through the whole
unrefList because it's not ordered anymore.

It is usually not large enough to have a significant impact on
performance because:
- Most of the time, the timers will be removed before unrefTimeout is
  called because their users (sockets mainly) cancel them when an I/O
  operation takes place.
- If they're not, it means that some I/O took a long time to happen, and
  the initiator of subsequents I/O operations that would add more timers
  has to wait for them to complete.

With this change, _unrefActive does not show as a significant
contributor in CPU profiling reports anymore.

Fixes #8160.
Minor style issues: `function() {` and two space indent, not four.  You could use assert.equal() here.