[QMS-624] fix BRouter mutex threading issues

[ntruchsess]

What is the linked issue for this pull request:

QMS-Maproom#624

What you have done:

replaced the mutex-protected field 'synchronous' by a reply-property that doesn't require to be protected as qt takes care when delivering via signal.

Steps to perform a simple smoke test:

[comment]: # N/A, you cannot see the difference in the ui

Does the code comply to the coding rules and naming conventions Coding Guidelines:

• yes

Is every user facing string in a tr() macro?

• yes

Did you add the ticket number and title into the changelog? Keep the numeric order in each release block.

• N/A as I'm raising this PR against existing PR [QMS-624] Suspicion: QMS crashes if BRouter RD5 file is missing Maproom/qmapshack#629 which would be included in the changelog.txt

[ntruchsess]

@kiozen while this is most likely not causing said crash it should be fixed as you are touching the affected code anyway