Store hostname hashes in memory rathen than the non-hashed host entri…

…es. Also assures that the host entries in known_hosts get saved in hashed format as it is currently standard in OpenSSH.

paramiko/hostkeys.py

@@ -141,6 +141,8 @@ def add(self, hostname, keytype, key):
             if (hostname in e.hostnames) and (e.key.get_name() == keytype):
                 e.key = key
                 return
+        if not hostname.startswith('|1|') and hash_hostname:
+            hostname = self.hash_host(hostname)
         self._entries.append(HostKeyEntry([hostname], key))
 
     def load(self, filename):