Logcontexts for replication command handlers

Run the handlers for replication commands as background processes. This should
improve the visibility in our metrics, and reduce the number of "running db
transaction from sentinel context" warnings.

Ideally it means converting the things that fire off deferreds into the night
into things that actually return a Deferred when they are done. I've made a bit
of a stab at this, but it will probably be leaky.

synapse/replication/tcp/commands.py

@@ -59,6 +59,12 @@ class Command(object):
         """
         return self.data
 
+    def get_logcontext_id(self):
+        """Get a suitable string for the logcontext when processing this command"""
+
+        # by default, we just use the command name.
+        return self.NAME
+
 
 class ServerCommand(Command):
     """Sent by the server on new connection and includes the server_name.
@@ -116,6 +122,9 @@ class RdataCommand(Command):
             _json_encoder.encode(self.row),
         ))
 
+    def get_logcontext_id(self):
+        return "RDATA-" + self.stream_name
+
 
 class PositionCommand(Command):
     """Sent by the client to tell the client the stream postition without
@@ -190,6 +199,9 @@ class ReplicateCommand(Command):
     def to_line(self):
         return " ".join((self.stream_name, str(self.token),))
 
+    def get_logcontext_id(self):
+        return "REPLICATE-" + self.stream_name
+
 
 class UserSyncCommand(Command):
     """Sent by the client to inform the server that a user has started or