Use SHA2-512 to store the core certificate digest in the client

Previously, MD5 was used.  Like the core password hashing update,
this implements a versioning system so that the digest will be
automagically upgraded without bugging the user.  If for some
reason the user downgrades again, the client will still work but
the user will be asked to validate the certificate again.

SHA2-256 is more common for storing certificate digests, but in
this case, SHA2-512 was used.  This is because Qt4 does not
implement any hash higher than SHA1 and a SHA2-512 implementation
is already bundled for use with the core password hashing.