persist the first slack thread message

[mkaczanowski]

TL;DR

Whenever there is upgrade the new slack thread is created. Slack API requires to provide a parent message ts for every child message.
The logger detects first message for a height and appends the parent ts to the following messages.

But the information about the parent message is not persisted in local store, so whenever blazar restarts... the new thread is potentially created (for the same upgrade)