[lib] implement removing OutboundP2P messages after confirmation

Summary:
After other peer process (backup message) we can remove it.
We track the order so when removing we can remove all older messages.

Tunnelbroker adds its own timestamp to message see here so we have to extract it. This is simple but moving this to function with descriptive name to improve readability.

Depends on D12077, D12080

Test Plan: Send confirmation and making sure that the message is removed

Reviewers: tomek, bartek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12111