message-tags / message-id support

[Half-Shot]

Spec https://ircv3.github.io/specs/extensions/message-ids.html

I'd really love to see message-ids land as it would really improve the reliability and efficiency of the matrix bridge. Having unique IDs for the message would allow for us to easily reject duplicate messages coming into the bridge from multiple clients which is an issue we have today (see ref). At the moment we solve this by having a single IRC TCP connection be responsible for a single channel and hashing the message strings together to ensure some sort of uniqueness, however this has issues if the client responsible for the channel is on the wrong side of a net-split, or is experiencing a delay. Furthermore, it does have it's own memory cost.

Having message IDs attached to messages means we can operate on a first come, first serve model for incoming messages which means that we just throw away duplicates as they arrive and won't need to worry about slow clients or splits for the most part.

Ref https://github.com/matrix-org/matrix-appservice-irc/blob/develop/src/irc/IrcEventBroker.ts#L17-L83

[idarlund]

This also opens up the ability for IRC clients to be able to use threads (https://ircv3.net/specs/client-tags/reply).