message bubble timestamp sometimes updated too late

[r10s]

sometimes, message bubble timestamp are updated too late:

• the timestamps in the message bubbles are updated every minute by a timer

• the timer seems to be stopped when the app goes to background and the app gets suspended; this stopping is probably okay

• when the app comes back from suspension, the timer is started - however not executed immediately, but only after a minute

-> this leads to delayed update; the app shows outdated timestamps for a minute.

things are fine when the app gets exited in between - or if the chat is opened.

things are also fine for all cases for the chatlist, the issue at hand seems to be fixed explicitly at https://github.com/deltachat/deltachat-ios/pull/1554