fix: Call send_sync_msg() only from the SMTP loop (#5780)

`Context::send_sync_msg()` mustn't be called from multiple tasks in parallel to avoid sending the
same sync items twice because sync items are removed from the db only after successful
sending. Let's guarantee this by calling `send_sync_msg()` only from the SMTP loop. Before
`send_sync_msg()` could be called in parallel from the SMTP loop and another task doing
e.g. `chat::sync()` which led to `test_multidevice_sync_chat` being flaky because of events
triggered by duplicated sync messages.

deltachat-rpc-client/tests/test_something.py

@@ -103,12 +103,12 @@ def test_account(acfactory) -> None:
     assert alice.get_chatlist(snapshot=True)
     assert alice.get_qr_code()
     assert alice.get_fresh_messages()
-    assert alice.get_next_messages()
 
     # Test sending empty message.
     assert len(bob.wait_next_messages()) == 0
     alice_chat_bob.send_text("")
     messages = bob.wait_next_messages()
+    assert bob.get_next_messages() == messages
     assert len(messages) == 1
     message = messages[0]
     snapshot = message.get_snapshot()