Mostly-working Hush full node sans Verus!!!

Every line of Verus-specific code has been removed from the codebase.
This code compiles on Linux and can do a partial sync. A full sync
and other extensive tests need to be done before it's merged into
the duke branch.

BUGS:

One known bug is that the node starts to CPU mine by default, lol.

src/keystore.h

@@ -25,7 +25,6 @@
 #include "pubkey.h"
 #include "script/script.h"
 #include "script/standard.h"
-#include "script/script_ext.h"
 #include "sync.h"
 #include "zcash/Address.hpp"
 #include "zcash/NoteEncryption.hpp"