Cache public keys for the duration of the agent

This saves a lot of time when connecting to multiple hosts
simultaneously (e.g., during a deploy) as every time we are asked to sign a
challenge, all public keys are iterated to find the correct one. This
can become especially slow when using the Bridge transport and/or many
identities are defined.

trezor_agent/__main__.py

@@ -147,8 +147,9 @@ class JustInTimeConnection(object):
         """Create a JIT connection object."""
         self.conn_factory = conn_factory
         self.identities = identities
+        self.public_keys = util.memoize(self._public_keys)  # a simple cache
 
-    def public_keys(self):
+    def _public_keys(self):
         """Return a list of SSH public keys (in textual format)."""
         conn = self.conn_factory()
         return [conn.get_public_key(i) for i in self.identities]