fix: Mailbox out cnt value

[whoTBecker]

Set cnt value just before a mailbox message is sent.
Since mailbox messages may sent in different order than the messages were claimed, the correct cnt value can even be determined at the time, when the message is sent out.

Adjust bitfield for cnt value within mailbox header structure.

Reset mailbox out cnt value when mailbox is stopped.

( Also included: Pull Request #194 )

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

❌ whoTBecker
❌ whoMBadura
_{You have signed the CLA already but the status is still pending? Let us recheck it.}