test: Separated query and loading internal sleep configs

To make schema cache loading wait tests robust it is necessary to provide three separate internal config variables:
* "internal-schema-cache-query-sleep" - introduces delay in schema queries execution
* "internal-schema-cache-load-sleep" - introduces delay between schema queries execution and processing their results
* "internal-schema-cache-relationship-load-sleep" - introduces delay in processing relationship query results

Thanks to these changes it is now possible to test various schema loading scenarios with the right granularity robustly (eg. make sure requests wait for schema loading but not for relationship loading).

test/io/test_io.py

@@ -825,11 +825,11 @@ def test_admin_ready_includes_schema_cache_state(defaultenv, metapostgrest):
         **defaultenv,
         "PGUSER": role,
         "PGRST_DB_ANON_ROLE": role,
-        "PGRST_INTERNAL_SCHEMA_CACHE_SLEEP": "500",
+        "PGRST_INTERNAL_SCHEMA_CACHE_QUERY_SLEEP": "500",
     }
 
     with run(env=env) as postgrest:
-        # The schema cache query takes at least 500ms, due to PGRST_INTERNAL_SCHEMA_CACHE_SLEEP above.
+        # The schema cache query takes at least 500ms, due to PGRST_INTERNAL_SCHEMA_CACHE_QUERY_SLEEP above.
         # Make it impossible to load the schema cache, by setting statement timeout to 400ms.
         set_statement_timeout(metapostgrest, role, 400)
 
@@ -855,11 +855,11 @@ def test_metrics_include_schema_cache_fails(defaultenv, metapostgrest):
     env = {
         **defaultenv,
         "PGUSER": role,
-        "PGRST_INTERNAL_SCHEMA_CACHE_SLEEP": "50",
+        "PGRST_INTERNAL_SCHEMA_CACHE_QUERY_SLEEP": "50",
     }
 
     with run(env=env) as postgrest:
-        # The schema cache query takes at least 20ms, due to PGRST_INTERNAL_SCHEMA_CACHE_SLEEP above.
+        # The schema cache query takes at least 20ms, due to PGRST_INTERNAL_SCHEMA_CACHE_QUERY_SLEEP above.
         # Make it impossible to load the schema cache, by setting statement timeout to 100ms.
         set_statement_timeout(metapostgrest, role, 20)
 
@@ -1320,7 +1320,7 @@ def test_schema_cache_concurrent_notifications(slow_schema_cache_env):
     "schema cache should be up-to-date whenever a notification is sent while another reload is in progress, see https://github.com/PostgREST/postgrest/issues/2791"
 
     internal_sleep = (
-        int(slow_schema_cache_env["PGRST_INTERNAL_SCHEMA_CACHE_SLEEP"]) / 1000
+        int(slow_schema_cache_env["PGRST_INTERNAL_SCHEMA_CACHE_QUERY_SLEEP"]) / 1000
     )
 
     with run(env=slow_schema_cache_env, wait_for_readiness=False) as postgrest: