KeepKey / Trezor: client split

We're going to want to do a few things differently, such as
device recovery.  So move the client code to clientbase.py
and create a per-plugin client.py file for the derived client
class.

plugins/keepkey/keepkey.py

@@ -1,4 +1,3 @@
-from ..trezor.client import trezor_client_class
 from ..trezor.plugin import TrezorCompatiblePlugin, TrezorCompatibleWallet
 
 
@@ -13,8 +12,7 @@ class KeepKeyPlugin(TrezorCompatiblePlugin):
     minimum_firmware = (1, 0, 0)
     wallet_class = KeepKeyWallet
     try:
-        from keepkeylib.client import proto, BaseClient, ProtocolMixin
-        client_class = trezor_client_class(ProtocolMixin, BaseClient, proto)
+        from .client import KeepKeyClient as client_class
         import keepkeylib.ckd_public as ckd_public
         from keepkeylib.client import types
         from keepkeylib.transport_hid import HidTransport, DEVICE_IDS