WIP: multi-relay routing

[link2xt]

This way we can place whatever we want into signature.

Closes #7865 once finished, the idea is to put all relay addresses into notation subpacket of the primary user ID signature.

TODO:

• Fix tests/test_multitransport.py::test_download_on_demand. It is currently broken because receiving the same pre-message second time results in trashing the second pre-message and replacing first pre-message, so essentially the message that already existed as a pre-message gets trashed. Then you get problem from Post-message resulted in receive_imf failure #7872 when post-message download is automatically requested, but pre-message got trashed.
• Bring back all the commented out signature subpackets or decide to drop them. SEIPDv2 feature is needed, we also want to recognize it and start sending SEIPDv2 if everyone supports it. Preferred algorithms and hashes practically just increase the signature size at the moment.
• Have some way to update contact public key (aka OpenPGP certificate). Currently (as of 2.44.0) we never update public keys from Autocrypt header. Simple way to fix this is to accept updates, but only from the signed messages and remember the timestamp, so it is impossible to e.g. forge an update with dropped encryption subkey. But we also want to update from Autocrypt-Gossip, this will need more careful merging, e.g. keeping old certificate but replacing user ID with the one that has highest timestamp on the signature. Currently I just trust the updates from Autocrypt header. EDIT: implemented certificate merging
• Merge certificates when vCard is imported as well.
• Only update the signature timestamp when actual changes happen to avoid leaking app start time, save the timestamp somewhere in the config
• Have some limit on the number of relays we take from the key, take no more than two or three relays.
• Online Python test that after removing primary relay user can still receive messages. Currently it works already but message is assigned to ad hoc group because the message is not recognized as sent to us. This is supposed to work via "intended recipient fingerprint" (e.g. fix: receive_imf: Look up key contact by intended recipient fingerprint (#7661) #7786) but something does not work. The problem is apparently caused by not stopping IMAP loop when deleting the transport.
• Per-context lock around prefetching (IMAP loop) or around receive_imf. Currently multitransport tests with download on demand fail because two IMAP loops receive the same messages and download them in parallel and this is not handled properly.