Refactoring the HAL backend fstab handling to make it more useful.
We now keep a cache of all the information found in the fstab.
It's updated on demand only if more than 10 seconds elapsed since the last update.
This way we don't introduce a polling, and we also make sure that the information used is not too old.

Opens the door to making some of our heuristics in the HAL backend for ignoring devices more efficient.