client.py: rename "server key" to "node key", use old name if present

This prepares for invitation-based reciprocal-permission Accounting. In
the scheme I'm developing, nodes publish "I accept shares from Y"
messages, which are assembled into a graph, and server will accept
shares from any client node reachable in this graph. For this to work,
the serverX->clientY edge must be connectable to the serverY->clientZ
edge, which means "clientY" and "serverY" must be connected. If clientY
and serverY are two distinct keys, they must be cross-signed. Life is
easier if there's just one key "Y", rather than distinct client- and
server- keys. Calling this one key "server.privkey" would be confusing.
"node.privkey" and "node.pubkey" makes more sense.

One-server-per-node is a pretty easy restriction. Originally I was
thinking that the client.key should be provided in each webapi call,
just like a filecap is, making a single node useable by multiple users
(Accounting principals), and not providing any ambient storage
authority. But I've been unable to think of a comfortable WUI for
that (at least without requiring javascript), nor a friendly way to
transfer account authority (e.g. writecaps that include storage
authority). So I'm more willing to have one-client-per-node these days.

(and note that this rename doesn't seriously preclude
many-clients-per-node or zero-clients-per-node anyways, it just makes
one-client-per-node less awkward)

src/allmydata/client.py

@@ -198,15 +198,20 @@ class Client(node.Node, pollmixin.PollMixin):
         self.convergence = base32.a2b(convergence_s)
         self._secret_holder = SecretHolder(lease_secret, self.convergence)
 
-    def _maybe_create_server_key(self):
+    def _maybe_create_node_key(self):
         # we only create the key once. On all subsequent runs, we re-use the
         # existing key
         def _make_key():
             sk_vs,vk_vs = keyutil.make_keypair()
             return sk_vs+"\n"
-        sk_vs = self.get_or_create_private_config("server.privkey", _make_key)
+        # for a while (between releases, before 1.10) this was known as
+        # server.privkey, but now it lives in node.privkey. This fallback can
+        # be removed after 1.10 is released.
+        sk_vs = self.get_private_config("server.privkey", None)
+        if not sk_vs:
+            sk_vs = self.get_or_create_private_config("node.privkey", _make_key)
         sk,vk_vs = keyutil.parse_privkey(sk_vs.strip())
-        self.write_config("server.pubkey", vk_vs+"\n")
+        self.write_config("node.pubkey", vk_vs+"\n")
         self._server_key = sk
 
     def _init_permutation_seed(self, ss):
@@ -236,7 +241,7 @@ class Client(node.Node, pollmixin.PollMixin):
             return
         readonly = self.get_config("storage", "readonly", False, boolean=True)
 
-        self._maybe_create_server_key()
+        self._maybe_create_node_key()
 
         storedir = os.path.join(self.basedir, self.STOREDIR)