Concatenated SMS handling has a nasty race condition that drops messages on the floor (!)

[eeeeeta]

If we process multiple CSMS parts in parallel, two attempts to set the room state can race with each other, meaning that we clobber one of the CSMS parts with the other :(

To be honest, the messages aren't dropped on the floor, because this is Matrix; we should be able to get at them with some room state poking. Still, not fun.