Refactor matrixfederationclient to fix logging (#3906)

We want to wait until we have read the response body before we log the request
as complete, otherwise a confusing thing happens where the request appears to
have completed, but we later fail it.

To do this, we factor the salient details of a request out to a separate
object, which can then keep track of the txn_id, so that it can be logged.

tests/server.py

@@ -280,3 +280,84 @@ def get_clock():
     clock = ThreadedMemoryReactorClock()
     hs_clock = Clock(clock)
     return (clock, hs_clock)
+
+
+@attr.s
+class FakeTransport(object):
+    """
+    A twisted.internet.interfaces.ITransport implementation which sends all its data
+    straight into an IProtocol object: it exists to connect two IProtocols together.
+
+    To use it, instantiate it with the receiving IProtocol, and then pass it to the
+    sending IProtocol's makeConnection method:
+
+        server = HTTPChannel()
+        client.makeConnection(FakeTransport(server, self.reactor))
+
+    If you want bidirectional communication, you'll need two instances.
+    """
+
+    other = attr.ib()
+    """The Protocol object which will receive any data written to this transport.
+
+    :type: twisted.internet.interfaces.IProtocol
+    """
+
+    _reactor = attr.ib()
+    """Test reactor
+
+    :type: twisted.internet.interfaces.IReactorTime
+    """
+
+    disconnecting = False
+    buffer = attr.ib(default=b'')
+    producer = attr.ib(default=None)
+
+    def getPeer(self):
+        return None
+
+    def getHost(self):
+        return None
+
+    def loseConnection(self):
+        self.disconnecting = True
+
+    def abortConnection(self):
+        self.disconnecting = True
+
+    def pauseProducing(self):
+        self.producer.pauseProducing()
+
+    def unregisterProducer(self):
+        if not self.producer:
+            return
+
+        self.producer = None
+
+    def registerProducer(self, producer, streaming):
+        self.producer = producer
+        self.producerStreaming = streaming
+
+        def _produce():
+            d = self.producer.resumeProducing()
+            d.addCallback(lambda x: self._reactor.callLater(0.1, _produce))
+
+        if not streaming:
+            self._reactor.callLater(0.0, _produce)
+
+    def write(self, byt):
+        self.buffer = self.buffer + byt
+
+        def _write():
+            if getattr(self.other, "transport") is not None:
+                self.other.dataReceived(self.buffer)
+                self.buffer = b""
+                return
+
+            self._reactor.callLater(0.0, _write)
+
+        _write()
+
+    def writeSequence(self, seq):
+        for x in seq:
+            self.write(x)