Add Validator Manifests (RIPD-772):

A Validator Manifest allows validators to use a generated ed25519
secret key as a master key for generating new validator public/secret
key pairs. Using this mechanism, rippled instances trust the master
ed25519 public key instead of the now-ephemeral validator public key.

Through a new message and propagation scheme, this lets a validator
change its ephemeral public key without requiring that all rippled
instances on the network restart after maintaining the configuration
file.

src/ripple/protocol/impl/HashPrefix.cpp

@@ -34,5 +34,6 @@ HashPrefix const HashPrefix::txSign                      ('S', 'T', 'X');
 HashPrefix const HashPrefix::txMultiSign                 ('S', 'M', 'T');
 HashPrefix const HashPrefix::validation                  ('V', 'A', 'L');
 HashPrefix const HashPrefix::proposal                    ('P', 'R', 'P');
+HashPrefix const HashPrefix::manifest                   ('M', 'A', 'N');
 
 } // ripple