Use a shared device manager

Use a shared device manager across USB devices (not yet taken
advantage of by ledger).  This reduces USB scans and abstracts
device management cleanly.

We no longer scan at regular intervals in a background thread.

plugins/trezor/client.py

@@ -77,7 +77,7 @@ def trezor_client_class(protocol_mixin, base_client, proto):
             self.msg_code_override = None
 
         def __str__(self):
-            return "%s/%s/%s" % (self.label(), self.device_id(), self.path[0])
+            return "%s/%s/%s" % (self.label(), self.device_id(), self.path)
 
         def label(self):
             '''The name given by the user to the device.'''
@@ -91,6 +91,9 @@ def trezor_client_class(protocol_mixin, base_client, proto):
             '''True if initialized, False if wiped.'''
             return self.features.initialized
 
+        def pair_wallet(self, wallet):
+            self.wallet = wallet
+
         def handler(self):
             assert self.wallet and self.wallet.handler
             return self.wallet.handler
@@ -111,6 +114,15 @@ def trezor_client_class(protocol_mixin, base_client, proto):
                 path.append(abs(int(x)) | prime)
             return path
 
+        def first_address(self, wallet, derivation):
+            assert not self.wallet
+            # Assign the wallet so we have a handler
+            self.wallet = wallet
+            try:
+                return self.address_from_derivation(derivation)
+            finally:
+                self.wallet = None
+
         def address_from_derivation(self, derivation):
             return self.get_address('Bitcoin', self.expand_path(derivation))
 
@@ -128,6 +140,24 @@ def trezor_client_class(protocol_mixin, base_client, proto):
             finally:
                 self.msg_code_override = None
 
+        def clear_session(self):
+            '''Clear the session to force pin (and passphrase if enabled)
+            re-entry.  Does not leak exceptions.'''
+            self.print_error("clear session:", self)
+            try:
+                super(TrezorClient, self).clear_session()
+            except BaseException as e:
+                # If the device was removed it has the same effect...
+                self.print_error("clear_session: ignoring error", str(e))
+                pass
+
+        def close(self):
+            '''Called when Our wallet was closed or the device removed.'''
+            self.print_error("disconnected")
+            self.clear_session()
+            # Release the device
+            self.transport.close()
+
         def firmware_version(self):
             f = self.features
             return (f.major_version, f.minor_version, f.patch_version)