Addressing persistent storage for activity queue

[golfinq]

This is a draft PR which attempts to address #31. There are still some TODOs and I am also looking for any feedback on the current method I am using

TODO:

• Formatting
• Prevent duplicates from being sent
• Determine who should handle recovery after a crash, and how that should be done
• Delete the Send Requests on disk after use
• Log errors returned by the potential database interface as opposed to ignoring them
• Actually test the code

[Nutomic]

The activity queue was removed in https://github.com/LemmyNet/activitypub-federation-rust/pull/75, closing this.