Only HashPrefix can create prefixes.

Have compiler enforce that we have our complete list of prefixes in HashPrefix.cpp by disallowing the rest of rippled to construct or modify them.

* Make constructor private.
* Disallow assignment.

@scottschurr, @ximinez
I'm assuming there's a place in the code that was using the (previously invisible) copy constructor?  If not, I'd be inclined to give that an "= delete" as well.