compute_ctl: Streamline and Pipeline startup SQL (#9717)

Before, compute_ctl didn't have a good registry for what command would
run when, depending exclusively on sync code to apply changes. When
users have many databases/roles to manage, this step can take a
substantial amount of time, breaking assumptions about low (re)start
times in other systems.

This commit reduces the time compute_ctl takes to restart when changes
must be applied, by making all commands more or less blind writes, and
applying these commands in an asynchronous context, only waiting for
completion once we know the commands have all been sent.

Additionally, this reduces time spent by batching per-database
operations where previously we would create a new SQL connection for
every user-database operation we planned to execute.

compute_tools/src/sql/default_grants.sql

@@ -0,0 +1,30 @@
+DO
+$$
+    BEGIN
+        IF EXISTS(
+            SELECT nspname
+            FROM pg_catalog.pg_namespace
+            WHERE nspname = 'public'
+        ) AND
+           current_setting('server_version_num')::int / 10000 >= 15
+        THEN
+            IF EXISTS(
+                SELECT rolname
+                FROM pg_catalog.pg_roles
+                WHERE rolname = 'web_access'
+            )
+            THEN
+                GRANT CREATE ON SCHEMA public TO web_access;
+            END IF;
+        END IF;
+        IF EXISTS(
+            SELECT nspname
+            FROM pg_catalog.pg_namespace
+            WHERE nspname = 'public'
+        )
+        THEN
+            ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT ALL ON TABLES TO neon_superuser WITH GRANT OPTION;
+            ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT ALL ON SEQUENCES TO neon_superuser WITH GRANT OPTION;
+        END IF;
+    END
+$$;