This PR has fixes for a number of instances in the bor heimdall stage
where nil headers are either ignored or inadvertently processed.
It also has a demotion of milestone related logging messages to debug
for missing blocks because the process is not at the head of the chain +
a general reduction in periodic logging to 30 secs rather than 20 to
reduce the log output on long runs.
In addition there is a refactor of persistValidatorSets to perform
validator set initiation in a seperate function. This is intended to
clarify the operation of persistValidatorSets - which is till performing
2 actions, persisting the snapshot and then using it to check the header
against synthesized validator set in the snapshot.
I using `https://heimdall-api-testnet.polygon.technology/` and seems
5sec timeout is not enough sometime - even that remote service working
well (node syncing well)
most of timeouts comes from same endpoint:
```
[bor.heimdall] request canceled reason="context deadline exceeded" path=/milestone/lastNoAck attempt=2
```
