Await un-partial-stating after a partial-state join (#12399)

When we join a room via the faster-joins mechanism, we end up with "partial
state" at some points on the event DAG. Many parts of the codebase need to
wait for the full state to load. So, we implement a mechanism to keep track of
which events have partial state, and wait for them to be fully-populated.

docker/complement/conf/homeserver.yaml

@@ -103,8 +103,10 @@ experimental_features:
   spaces_enabled: true
   # Enable history backfilling support
   msc2716_enabled: true
-  # server-side support for partial state in /send_join
+  # server-side support for partial state in /send_join responses
   msc3706_enabled: true
+  # client-side support for partial state in /send_join responses
+  faster_joins: true
   # Enable jump to date endpoint
   msc3030_enabled: true