timer: cache the minimal value.

timer: cache the minimal value.

We spend a lot of time searching for the next timer to expire: by caching
the minimum, we can skip most of this work.  Even if timers are deleted,
the minimum will be a starting point for searching.

The expected-usage benchmark has to be increased by a factor of 100,
otherwise it's now too short.

Before:
	$ ./expected-usage
	1000000 in 12.701647935

After:
	$ ./expected-usage  1000000
	1000000 in 0.061095153

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>