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.
Nit: "Prune old blocks" may be an easier explanation for the flag.