archive-push breaks pgbadger

Reported by @triwada and  Mikhail Kulagin.
pbk now prints messages in PostgreSQL log in the following format:
```
$timestamp [$pid]: [$threadnum-1]: $elevel: text
```
Example:
```
2020-06-22 16:20:00 MSK [20014]: [1-1]: INFO: pg_probackup archive-push completed successfully, pushed: 1, skipped: 0, time elapsed: 108ms
```