R1:bbb321395a96

Support Aphlict clustering

Summary:
Ref T6915. This allows multiple notification servers to talk to each other:

- Every server has a list of every other server, including itself.
- Every server generates a unique fingerprint at startup, like "XjeHuPKPBKHUmXkB".
- Every time a server gets a message, it marks it with its personal fingerprint, then sends it to every other server.
- Servers do not retransmit messages that they've already seen (already marked with their fingerprint).
- Servers learn…