send data to server as bytes

This is a bit of a punt, especially WRT extended info.  For regular
ZEO servers, FileStorage is going to want to pickle extended info, but
byteserver can't pickle extended info. I'm not sure how best to resolve this.

src/ZEO/ClientStorage.py

@@ -793,7 +793,7 @@ def tpc_vote(self, txn):
     def tpc_transaction(self):
         return self._transaction
 
-    def tpc_begin(self, txn, tid=None, status=' '):
+    def tpc_begin(self, txn, tid=None, status=b' '):
         """Storage API: begin a transaction."""
         if self._is_read_only:
             raise POSException.ReadOnlyError()
@@ -823,7 +823,10 @@ def tpc_begin(self, txn, tid=None, status=' '):
         try:
             self._async(
                 'tpc_begin', id(txn),
-                txn.user, txn.description, txn._extension, tid, status)
+                txn.user.encode('utf-8'),
+                txn.description.encode('utf-8'),
+                b'', # TODO: deal with extension info
+                tid, status)
         except ClientDisconnected:
             self.tpc_end(txn)
             raise