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This allows us not to create a new SQLite connection from a potentially
different VLC thread for each media

This saves roughly 200/300ms during startup

Instead of having a per-field SQL request.

Even though it does exactly the same as as executeUpdate/executeDelete,
it improves clarity

Reading from the DB is "cheap", and the cache hurts the maintainability
of the code.
This removes the CachePolicy and CacheColumn field from TablePolicy.
Instead, a PrimaryKeyColumn has been added to TablePolicy

Only use a Media instance & expose media deletion methods internally
only

Instead, have the caller specify which type they want.
This allows us avoid using those ugly std::static_pointer_cast

Use duck typing to assert that the entity will have an id() method

This is lighter, and allows us to use the interface type instead of
the implementation type as the key() method parameter.