Implement TorProvider class as described by Brian Warner

https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2490#comment:10

src/allmydata/tor.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+
+from __future__ import absolute_import
+from __future__ import print_function
+from __future__ import with_statement
+
+from twisted.internet import reactor, defer
+
+from txtorcon import torconfig
+from txtorcon import torcontrolprotocol
+
+
+class TorProvider:
+
+    def __init__(self, tor_binary=None, control_endpoint=None):
+        assert tor_binary is not None or control_endpoint is not None
+
+        self.tor_binary = tor_binary
+        self.control_endpoint = control_endpoint
+        self.tor_control_protocol = None
+
+    def get_control_protocol(self):
+        """
+        Returns a deferred which fires with the txtorcon tor control port object
+        """
+        if self.tor_control_protocol is not None:
+            d = defer.succeed(self.tor_control_protocol)
+        else:
+            if self.control_endpoint is None:
+                config = torconfig.TorConfig()
+                d = torconfig.launch_tor(config, reactor, tor_binary=self.tor_binary)
+                def remember_tor_protocol(result):
+                    self.tor_control_protocol = result.tor_protocol
+                    return result.tor_protocol
+                d.addCallback(remember_tor_protocol)
+            else:
+                d = torcontrolprotocol.connect(self.control_endpoint) # XXX use a password_function?
+                def remember_tor_protocol(result):
+                    self.tor_control_protocol = result
+                    return result
+                d.addCallback(remember_tor_protocol)
+        return d