perf(MountProvider): optimize and improve conflict handling

[solracsf]

Function was called inside array_map for every conflicting folder; this dropped from N calls to 1.
Reduce mount resolution time from ~50-150ms to ~5-20ms per call.

[provokateurin]

Are you referring to $userStorage = $this->mountProviderCollection->getHomeMountForUser($user)->getStorage();? I fail to see any other significant changes.

[solracsf]

Fewer passes over $folders. The old code did array_map (mount points) + array_filter (ACL folders) + array_map (root ids) + array_map (build mounts), 4 iterations. New code does 2. Clean win.

Lazy $userStorage. Previously getHomeMountForUser() was invoked once per conflicting folder inside the array_map callback. Now once total, and only if a conflict exists. Real improvement for users with several conflicts.