Run push_receipts_to_remotes as background job (#4707)

I suspect the CPU usage metrics for this are going to /dev/null at the moment.

changelog.d/4707.misc

@@ -0,0 +1 @@
+Run push_receipts_to_remotes as background job.

synapse/handlers/receipts.py

@@ -16,8 +16,8 @@ import logging
 
 from twisted.internet import defer
 
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.types import get_domain_from_id
-from synapse.util import logcontext
 
 from ._base import BaseHandler
 
@@ -59,7 +59,9 @@ class ReceiptsHandler(BaseHandler):
         if is_new:
             # fire off a process in the background to send the receipt to
             # remote servers
-            self._push_remotes([receipt])
+            run_as_background_process(
+                'push_receipts_to_remotes', self._push_remotes, receipt
+            )
 
     @defer.inlineCallbacks
     def _received_remote_receipt(self, origin, content):
@@ -125,15 +127,13 @@ class ReceiptsHandler(BaseHandler):
 
         defer.returnValue(True)
 
-    @logcontext.preserve_fn   # caller should not yield on this
     @defer.inlineCallbacks
-    def _push_remotes(self, receipts):
-        """Given a list of receipts, works out which remote servers should be
+    def _push_remotes(self, receipt):
+        """Given a receipt, works out which remote servers should be
         poked and pokes them.
         """
         try:
-            # TODO: Some of this stuff should be coallesced.
-            for receipt in receipts:
+            # TODO: optimise this to move some of the work to the workers.
             room_id = receipt["room_id"]
             receipt_type = receipt["receipt_type"]
             user_id = receipt["user_id"]