[Store and forward] Private message storage and retrieval

[sdbondi]

Description

Storage and retreival of messages without an origin (PR #1686)
All private/anonymous messages are stored as low priority (short TTL)
messages.

• New response type Anonymous for store and forward query responses
• Added some more logging
• DHT store and forward integration test
• SAF messages can never be stored in store and forward

Motivation and Context

Since messages can be completely private (no known source or destination) SAF needs to store and relay these messages.

How Has This Been Tested?

Added a store and forward integration test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Feature refactor (No new feature or functional changes, but performance or technical debt improvements)
• New Tests
• Documentation

Checklist:

• I'm merging against the development branch.
• I ran cargo-fmt --all before pushing.
• I have squashed my commits into a single commit.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[CjS77]

LGTM