daemon: Prevent spawning new daemons if mount operation pending

A new daemon is always spawned if MountLocation method (or LookupMount for automounted) is called and the respective mount isn't registered yet. This is not usually an issue, because the redundant daemons are consequently removed. However, this is a problem if mount operations hang for some reason. This may happen e.g. with trash backend due to stale NFS mounts. Consequently, new and new daemons are spawned which may lead to system failures due to lack of system resources. See the following downstream bug report: https://bugzilla.redhat.com/show_bug.cgi?id=1632960

Let's fix that behavior simply by preventing spawning of new daemons if respective mount operations are already pending.