Lock plugin should also query child locks when checking if can lock

[PVince81]

Assuming Depth infinity. When setting an exclusive lock on "/parent", we must make sure that there is no exclusive lock on any children "/parent/*".

The LockPlugin does not query child locks here https://github.com/sabre-io/dav/blob/master/lib/DAV/Locks/Plugin.php#L171

Discovered here https://github.com/owncloud/core/issues/34358

@DeepDiver1975