Improve precision of timer ticks

[amelchio]

Description:

The timer tries to tick a whole number of seconds since it was started. Thus, if it is started very close to a second rolling over, many ticks will fire on the wrong second. This is because we are usually a little bit late with scheduled tasks. I added a log statement to show this.

2018-07-29 09:27:16 [...] Timer tick 2018-07-29 07:27:16.998479+00:00
2018-07-29 09:27:18 [...] Timer tick 2018-07-29 07:27:18.000322+00:00 <-- :17
2018-07-29 09:27:18 [...] Timer tick 2018-07-29 07:27:18.995209+00:00
2018-07-29 09:27:20 [...] Timer tick 2018-07-29 07:27:20.000136+00:00 <-- :19
2018-07-29 09:27:21 [...] Timer tick 2018-07-29 07:27:21.000139+00:00 <-- :20
2018-07-29 09:27:22 [...] Timer tick 2018-07-29 07:27:22.000105+00:00 <-- :21
2018-07-29 09:27:22 [...] Timer tick 2018-07-29 07:27:22.999665+00:00

To help avoid these double and missed ticks, this PR delays the first tick until a second rolls over. This means that we can be almost one second late and still work as expected.

This is just an improvement, more complete fixes are discussed in home-assistant/architecture#50.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

This is a good smaller version of the other PR.

[pvizeli]

I think on reset we need also fallback to slp_seconds = 1 - (dt_util.utcnow().microsecond / 10**6)

[amelchio]

Thanks, that’s right. I will make the PR.