[CUL transport] Fix for issue with CUL transport race condition

The PR #5329 breaks all maxcul installations (see #5753) because the credit10ms state is not updated anymore on start. It also changes the requestCreditReport method to prevent any race coditions. From my point of few it’s save to merge #5686 to get the old behaviour back without any race conditions. Everybody agrees?

After all issues are fixed we can start a new discussion about it’s better to explicitly get the credit report or just send a command and handle “credit errors”, but for the moment we should recover the old interface.