Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/storage/databases/main/schema/delta/58/19instance_map.sql.postgres

@@ -17,9 +17,9 @@
 -- A unique and immutable mapping between instance name and an integer ID. This
 -- lets us refer to instances via a small ID in e.g. stream tokens, without
 -- having to encode the full name.
-CREATE TABLE instance_map (
+CREATE TABLE IF NOT EXISTS instance_map (
     instance_id SERIAL PRIMARY KEY,
     instance_name TEXT NOT NULL
 );
 
-CREATE UNIQUE INDEX instance_map_idx ON instance_map(instance_name);
+CREATE UNIQUE INDEX IF NOT EXISTS instance_map_idx ON instance_map(instance_name);

synapse/storage/databases/main/stream.py

@@ -1226,7 +1226,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore, metaclass=abc.ABCMeta):
 
     @cached()
     async def get_id_for_instance(self, instance_name: str) -> int:
-        """Get a unique, immutable ID that corresponds to the instance.
+        """Get a unique, immutable ID that corresponds to the given Synapse worker instance.
         """
 
         def _get_id_for_instance_txn(txn):