Have replication clients remove _INT_STREAM_POS

Suppose worker A makes an internal http request from worker B. B may
make changes that A later learns about over replication. We want A's
request to block until it has seen those changes—mainly to ensure A's
caches are invalidated promptly. This helps provide read-after-write
consistency, eliminating entire categories of races and test flakes.

To implement this, B includes a top-level field `_INT_STREAM_POS` in its
response JSON. Roughly speaking, the field's value tells A what to wait
for. But we weren't removing that internal field before A's request
completed!

Introduced in #14820.
Fixes #15308.

synapse/replication/http/_base.py

@@ -345,7 +345,7 @@ async def send_request(*, instance_name: str = "master", **kwargs: Any) -> Any:
                 _outgoing_request_counter.labels(cls.NAME, 200).inc()
 
                 # Wait on any streams that the remote may have written to.
-                for stream_name, position in result.get(
+                for stream_name, position in result.pop(
                     _STREAM_POSITION_KEY, {}
                 ).items():
                     await replication.wait_for_stream_position(