factor out "Tub maker"

This adds Node._create_tub(), which knows how to make a Tub with all the
right options and connection handlers that were specified in
tahoe.cfg (the connection handlers are disabled for now, but they'll get
implemented soon).

The new Node.create_main_tub() calls it. This main Tub is used:

* to connect to the Introducer
* to host the Helper (if enabled)
* to host the Storage Server (if enabled)

Node._create_tub() is also passed into the StorageFarmBroker, which
passes it into each NativeStorageServer, to create the (separate) Tub
for each server connection. _create_tub knows about the options, and
NativeStorageServer can override the connection handlers. This way we
don't need to pass tub options or default handlers into Client,
StorageFarmBroker, or NativeStorageServer.

A number of tests create NativeStorageServer objects: these were updated
to match the new arguments. test_storage_client was simplified because
we no longer need to mock out the Tub() constructor.

src/allmydata/client.py

@@ -353,9 +353,9 @@ class Client(node.Node, pollmixin.PollMixin):
         ps = self.get_config("client", "peers.preferred", "").split(",")
         preferred_peers = tuple([p.strip() for p in ps if p != ""])
         sb = storage_client.StorageFarmBroker(permute_peers=True,
+                                              tub_maker=self._create_tub,
                                               preferred_peers=preferred_peers,
-                                              tub_options=self.tub_options,
-                                              tub_handlers=self.tub_handlers)
+                                              )
         self.storage_broker = sb
         sb.setServiceParent(self)