This esp. speeds up receive_imf a bit when we recreate the member list (recreate_member_list == true).
It's a preparation for https://github.com/deltachat/deltachat-core-rust/issues/3768, which will be a one-line-fix, but recreate the member list more often, so that we want to optimize this case a bit.
It also adds a benchmark for this case. It's not that easy to make the benchmark non-flaky, but by closing all other programs and locking the CPU to 1.5GHz it worked. It is consistently a few percent faster than ./without-optim:
```
Receive messages/Receive 100 Chat-Group-Member-{Added|Removed} messages
time: [52.257 ms 52.569 ms 52.941 ms]
change: [-3.5301% -2.6181% -1.6697%] (p = 0.00 < 0.05)
Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
4 (4.00%) high mild
3 (3.00%) high severe
```
The state bob needs to maintain during a secure-join process when
exchanging messages used to be stored on the context. This means if
the process was killed this state was lost and the securejoin process
would fail. Moving this state into the database should help this.
This still only allows a single securejoin process at a time, this may
be relaxed in the future. For now any previous securejoin process
that was running is killed if a new one is started (this was already
the case).
This can remove some of the complexity around BobState handling: since
the state is in the database we can already make state interactions
transactional and correct. We no longer need the mutex around the
state handling. This means the BobStateHandle construct that was
handling the interactions between always having a valid state and
handling the mutex is no longer needed, resulting in some nice
simplifications.
Part of #2777.
