Speed up appservice transactions when sending them

This should be done at an earlier level such as where the transactions are calculated, however that doesn't solve the immediate backlog problem on t2bot.io

Instead, this alters the HTTP layer to short-circuit the transaction away from the appservice where possible to avoid delaying it.

synapse/appservice/api.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import time
 import urllib.parse
 from typing import TYPE_CHECKING, Any, Dict, Iterable, List, Mapping, Optional, Tuple
 
@@ -327,6 +328,14 @@ async def push_bulk(
                     "left": list(device_list_summary.left),
                 }
 
+        has_events = len(serialized_events) == 0
+        has_edus = service.supports_ephemeral and len(ephemeral) == 0
+        has_device_list_changes = service.msc3202_transaction_extensions and (len(list(device_list_summary.changed)) == 0 or len(list(device_list_summary.left)) == 0)
+        has_messages = service.supports_ephemeral and len(to_device_messages) == 0
+        if not has_events and not has_edus and not has_device_list_changes and not has_messages:
+            logger.info("Returning early on transaction: no events to send")
+            return True
+
         try:
             await self.put_json(
                 uri=uri,
@@ -368,7 +377,19 @@ async def push_bulk(
     def _serialize(
         self, service: "ApplicationService", events: Iterable[EventBase]
     ) -> List[JsonDict]:
+        new_events = []
         time_now = self.clock.time_msec()
+
+        for event in events:
+            if int(round(time.time() * 1000)) - event.origin_server_ts > (15 * 60 * 1000):
+                logger.warning("Dropping event (due to age) %s" % event.event_id)
+                continue
+            if service.id != "github" and service.is_interested_in_user(event.sender) and event.sender.endswith(":t2bot.io"):
+                logger.warning("Dropping event (due to echo) %s" % event.event_id)
+                continue
+            logger.info("Allowing @ fallback: %s" % event.event_id)
+            new_events.append(event)
+
         return [
             serialize_event(
                 e,
@@ -387,5 +408,5 @@ def _serialize(
                     ),
                 ),
             )
-            for e in events
+            for e in new_events
         ]