runtime: clean up timer state

The timers had evolved to the point where the state was stored as follows:

	if timer in heap:
	    state has timerHeaped set
	    if heap timer is stale:
	        heap deadline in t.when
	        real deadline in t.nextWhen
	        state has timerNextWhen set
	    else:
	        real deadline in t.when
	        t.nextWhen unset
	else:
	    real deadline in t.when
	    t.nextWhen unset

That made it hard to find the real deadline and just hard to think about everything.
The new state is:

	real deadline in t.when (always)
	if timer in heap:
	    state has timerHeaped set
	    heap deadline in t.whenHeap
	    if heap timer is stale:
	        state has timerModified set

Separately, the 'state' word itself was being used as a lock
and state bits because the code started with CAS loops,
which we abstracted into the lock/unlock methods step by step.
At this point, we can switch to a real lock, making sure to
publish the one boolean needed by timers fast paths
at each unlock.

All this simplifies various logic considerably.

Change-Id: I35766204f7a26d999206bd56cc0db60ad1b17cbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/570335
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

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