Misuse of TimestampDifference() in the autoprewarm feature of pg_prewarm

Hi Hackers,

Today I have accidentally noticed that autoprewarm feature of pg_prewarm
used TimestampDifference()'s results in a wrong way.

First, it used *seconds* result from it as a *milliseconds*. It was
causing it to make dump file autoprewarm.blocks ~every second with
default setting of autoprewarm_interval = 300s.

Here is a log part with debug output in this case:

```
2020-11-09 19:09:00.162 MSK [85328] LOG: dumping autoprewarm.blocks
2020-11-09 19:09:01.161 MSK [85328] LOG: dumping autoprewarm.blocks
2020-11-09 19:09:02.160 MSK [85328] LOG: dumping autoprewarm.blocks
2020-11-09 19:09:03.159 MSK [85328] LOG: dumping autoprewarm.blocks
```

After fixing this issue I have noticed that it still dumps blocks twice
at each timeout (here I set autoprewarm_interval to 15s):

```
2020-11-09 19:18:59.692 MSK [85662] LOG: dumping autoprewarm.blocks
2020-11-09 19:18:59.700 MSK [85662] LOG: dumping autoprewarm.blocks

2020-11-09 19:19:14.694 MSK [85662] LOG: dumping autoprewarm.blocks
2020-11-09 19:19:14.704 MSK [85662] LOG: dumping autoprewarm.blocks
```

This happens because at timeout time we were using continue, but
actually we still have to wait the entire autoprewarm_interval after
successful dumping.

I have fixed both issues in the attached patches and also added a
minimalistic tap test as a first one to verify that this automatic
damping still works after refactoring. I put Robert into CC, since he is
an author of this feature.

What do you think?

Regards
--
Alexey Kondratov

Postgres Professional https://www.postgrespro.com
Russian Postgres Company