Daemon: license cache flushing is too expensive

The current logic of flushing reservation contents to disk includes

• removing all existing files for the license (including other reservations)
• writing the license contents again
• writing all reservations' contents again

This makes the flushing exponentially more expensive, when new reservations are attached to or removed from license. Consider optimizing the flushing logic.

The visible effect of the current behavior is sub-optimal performance for certain license types, such as process based floating licenses when there is a high amount of clients requesting a license.