piroko

I’ve been running my own homeserver for over a year now. As long as I’m not joined to the “giant” rooms like #matrix:matrix.org and #riot:matrix.org, my user’s messages send instantly. Definitely recommend running your own server if you can manage it

Sounds like you're looking for https://matrix.org

Hits all your technical requirements. Setting up your own homeserver is cake. Federation is a key part of the core design. End to end encryption is just about to be finalized. History syncing between multiple clients. Bridges for pulling in other chat systems like slack and IRC. Completely open source. To me, this is the perfect messaging platform. Just needs some UI polish and I could see it really taking off. Had you seen this yet?