CachedBlockDevice::flush should flush LRU

[roblabla]

From @Orycterope 's comment

You must flush the LRU !
At least flush all dirty blocks in the LRU. You should also consider emptying out the LRU all together.
Document the behavior you chose to implement, to avoid shattered expectancies after days in gdb.

[Orycterope]

*expectations. Guess I was drunk that night while doing the review.