Shutdown/restart without losing messages

[leonerd]

Both Gitter and Matrix support the concept of "read markers", to let clients remember what messages they've got up to. We can probably use these on both sides to let the bridge catch up any missed messages while it was being restarted.

[leonerd]

Actually, the push-based AS hosting infrastructure already does this in the M->G direction; we just need to write some code to use the read markers on the Gitter side to make G->M behave the same.