medialib: acquire priority access

During indexation, the medialibrary executes a lot of write SQL queries from a background thread. As a consequence, requests initiated by VLC to retrieve data to show in the UI are excessively delayed (see 1).

These requests are not executed on the UI thread anymore (since 2), but it is still not acceptable to wait tens of seconds just to display local data.

Therefore, a new priority access mechanism has been implemented in medialibrary (3). Acquire this priority access for all Get and List requests initiated from VLC.