The benchmark function (e.g. `recv_all_emails()`) is executed multiple
times on the same context. During the second iteration, all the emails
were already in the database, so, receiving them again failed.
This PR fixes that by passing in a second `iteration` counter that is
different for every invocation of the benchmark function.
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
```
All contexts created by the same account manager
share stock string translations. Setting translation on
a single context automatically sets translations for all other
accounts, so it is enough to set translations on the active account.
This allows account manager to construct a single event channel and
inject it into all created contexts instead of aggregating events from
separate event emitters.
