Unstable error messages in services due to races related with service status vector

[pavel-zotov]

Ticked was created by Alex request. Failed restore (using services API) can miss final message "Exiting before completion due to errors".

We have QA test for CORE-5771. It verifies that attempt to restore DB via services when target DB exists does not cause FB crash.
Outcome if this test became unstable when ServerMode = SS since 6.0.0.100: message "-Exiting before completion due to errors" often is missed from expected output.
Illustration can be seen in attached screen.

[AlexPeshkoff]

Reopened in order to backport after running QA on v.6