Modules: ignore unknown and non-timer ids in clearTimeout()

Modules: ignore unknown and non-timer ids in clearTimeout()

Previously, clearTimeout() looked up the id among all pending native
events of the context, so a timer id coinciding with the id of
another event, such as a pending ngx.fetch() or a subrequest, removed
that event and freed its data.  In addition, an exception was thrown
for an unknown id.

Now, only timer events are removed, and unknown ids are ignored, as
required by the specification.

The issue was introduced in 9de30135 (0.8.3).