lib/events: Be more resilient against dropping events (fixes #3952)

Instead of just immediately dropping the event if the subscription isn't
ready to receive it, give it 15 ms to catch up. The value 15 ms is
grabbed out of thin air - it just seems reasonable to me.

The timer juggling makes the event send pretty much exactly twice as
slow as it was before, but we're still under a microsecond. I think it's
negligible compared to whatever event that just happened that we're
interested in logging (usually a file operation of some kind).

	benchmark                  old ns/op     new ns/op     delta
	BenchmarkBufferedSub-8     475           950           +100.00%

	benchmark                  old allocs     new allocs     delta
	BenchmarkBufferedSub-8     4              4              +0.00%

	benchmark                  old bytes     new bytes     delta
	BenchmarkBufferedSub-8     104           117           +12.50%

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3960