1. Allow timers to be freed in their callback.
2. Clarify in header that we have to terminate our timer with timer_finished()
eventually.
3. We don't currently have plugins with more than one outstanding timer, but
it certainly would be possible, so fix in_timer to be a counter.
Suggested-by: @ZmnSCPxj
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
