introducer.client: use integer seqnums, not time-based. Closes #1767.

This stores the sequence number in BASEDIR/announcement-seqnum, and
increments it each time any service is published (every service
announcement is regenerated with the new sequence number). As everyone
knows, time is an illusion, and occasionally goes backwards, so a
counter is generally safer (and reveals less information about the
node).

Later, we'll improve the introducer client to tolerate rollbacks (where,
perhaps due to a VM being restarted from an earlier checkpoint, the
stored sequence number reverts to an earlier version).

src/allmydata/client.py

@@ -163,13 +163,24 @@ class Client(node.Node, pollmixin.PollMixin):
         if webport:
             self.init_web(webport) # strports string
 
+    def _sequencer(self):
+        seqnum_s = self.get_config_from_file("announcement-seqnum")
+        if not seqnum_s:
+            seqnum_s = "0"
+        seqnum = int(seqnum_s.strip())
+        seqnum += 1 # increment
+        self.write_config("announcement-seqnum", "%d\n" % seqnum)
+        nonce = _make_secret().strip()
+        return seqnum, nonce
+
     def init_introducer_client(self):
         self.introducer_furl = self.get_config("client", "introducer.furl")
         ic = IntroducerClient(self.tub, self.introducer_furl,
                               self.nickname,
                               str(allmydata.__full_version__),
                               str(self.OLDEST_SUPPORTED_VERSION),
-                              self.get_app_versions())
+                              self.get_app_versions(),
+                              self._sequencer)
         self.introducer_client = ic
         # hold off on starting the IntroducerClient until our tub has been
         # started, so we'll have a useful address on our RemoteReference, so