Nonblocking master service (#94325)

The changes introduced in #92021 mean that there is no need for the
master service to block its thread while waiting for each publication to
complete. This commit removes the now-unnecessary blocking.

This commit also removes the now-unnecessary fake blocking in the
`FakeThreadPoolMasterService` used in tests, bringing the implementation
covered by tests closer to the production implementation.

Author: DaveCTurner