fix #3255

fix #3254

fix #3252

fix #3251

This makes us compatible with mam 6.0

fix #3249

Otherwise, some client won’t do MAM, since they don’t know biboumi supports
it.

And add a test for it.

fix #3238

ref #3231

And add tests for all the mode changes

fix #2886

As the doc says, the nick changes must be explicit in an already joined
room, and not when joining a room.

After printing the message, it calls “call_soon(run_scenario)”, and
sometimes, if a stanza handler has already been queued, then that handler
will be called BEFORE the “run_scenario” function could be run, and so no
stanza_checker had the possibility to be set.

fix #3171

fix #3216