Notification ttl

[lassemand]

Purpose

Adding threshold to when a notification is no longer considered valid enough to be sent

Changes

Single function which ensures catchup at start time

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[lassemand]

In general I am opposed to using inline comments unless absolutely necessary. This is because it removes readability of the actual source code. I try to do self explanatory code. If you think I need better named variables let me know. I have added docstring providing an overall explanation of the purpose of the method and tried to perform some optimisations on namings.