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.
CAutoFile already 'degenerates' into a  FILE*. Let's not add more magic behavior.

    operator FILE*()            { return file; }

I'd prefer an explicit method 'bool IsNull() const { return file==0; }'.