Modules: clear QuickJS hooks before a context is reused

Modules: clear QuickJS hooks before a context is reused

Previously, an exit hook installed during context teardown, for example
by a promise job queued by the exit hook, was kept in the context.  When
the context was reused, the hook was called during the teardown of a
later request.

Now hooks are cleared together with the pending events, after the last
user code of the request has run.

The issue was introduced in 04f6dfb9 (0.9.2).