Autoprune

This pull extends #4515 and #4481 implementing a lightweight sanity check which executes periodically during runtime to make sure the block files needed by the node are present.

There's also a new mode of operation introduced: -pruned, which automatically removes old block files and marks the blocks as pruned.

This should allow to lower the amount of resources needed to run a node.

See the individual commits, and the other pulls for details about all the changes introduced.
Being pruned is incompatible with a wallet, really. If you need to rescan or load a wallet that's older than the blocks you have on disk, you're going to have to download them and throw them out all over again. As I understand it, it's an implied requirement of having a wallet that you must be able to do a rescan as necessary.