Add members to chats in a single sql transation (#3780)

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
```

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Changes
 - Refactor: Remove the remaining AsRef<str> #3669
+- Small speedup #3780
 
 ### API-Changes
 - Add Python API to send reactions #3762