Move positions file to being bbolt instead of raw file

[mattdurham]

PR Description

Instead of using a raw file with xml which is prone to problems in the case of a hard reboot, switched to using bbolt. bbolt is the storage backend for etcd so is mature and has a team behind it. In testing it created a 32kb file, which looks to be the minimum file size even though its not storing very much text. This code also attempts to transition their old xml, assuming they are using the defaults.

Which issue(s) this PR fixes

Closes #4916

Notes to the Reviewer

This is a step to test out using a kv store for more usages beyond this singular usage.

PR Checklist

• CHANGELOG.md updated
• Tests updated

[rfratto]

Is there a simpler way of fixing #4916? On first glance this feels like killing a fly with a machine gun :) Other projects use renames to make the writes act as atomic operations, avoiding the corruption in #4916.

There may be good reasons to start adopting a KV store, but it has its own set of tradeoffs and may be heavier than is needed to strictly fix #4916.

[rfratto]

This is a trojan horse

You may want to avoid saying things like this :) This will alarm people; you're not writing malware here.

[mattdurham]

This is absolutely heavier than a rename, one of the primary goals is for me to get some operational knowledge and see how we can use a kv elsewhere. Made my trojan horse comment more clear.