Remove option to skip locking of tables during emulated upserts (#14469)

To perform an emulated upsert into a table safely, we must either:
 * lock the table,
 * be the only writer upserting into the table
 * or rely on another unique index being present.

When the 2nd or 3rd cases were applicable, we previously avoided locking
the table as an optimization. However, as seen in #14406, it is easy to
slip up when adding new schema deltas and corrupt the database.

The only time we lock when performing emulated upserts is while waiting
for background updates on postgres. On sqlite, we do no locking at all.

Let's remove the option to skip locking tables, so that we don't shoot
ourselves in the foot again.

Signed-off-by: Sean Quah <seanq@matrix.org>

synapse/storage/databases/main/devices.py

@@ -1744,9 +1744,6 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
                 table="device_lists_remote_cache",
                 keyvalues={"user_id": user_id, "device_id": device_id},
                 values={"content": json_encoder.encode(content)},
-                # we don't need to lock, because we assume we are the only thread
-                # updating this user's devices.
-                lock=False,
             )
 
         txn.call_after(self._get_cached_user_device.invalidate, (user_id, device_id))
@@ -1760,9 +1757,6 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             table="device_lists_remote_extremeties",
             keyvalues={"user_id": user_id},
             values={"stream_id": stream_id},
-            # again, we can assume we are the only thread updating this user's
-            # extremity.
-            lock=False,
         )
 
     async def update_remote_device_list_cache(
@@ -1815,9 +1809,6 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             table="device_lists_remote_extremeties",
             keyvalues={"user_id": user_id},
             values={"stream_id": stream_id},
-            # we don't need to lock, because we can assume we are the only thread
-            # updating this user's extremity.
-            lock=False,
         )
 
     async def add_device_change_to_streams(