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.
Perhaps use ``GetBlockPosFilename(pos, "blk")`` also for the log message? You could perhaps also merge these 2 functions into one by just passing a flag for undo or normal block file, as most code is a duplicate.