fix: schema cache retrying without backoff

Fixes https://github.com/PostgREST/postgrest/issues/3523.

Now if there's a failure when obtaining the pg version OR schema cache,
we do the same retrying process. This way we don't add two retries.

Refactors and renames the "connectionWorker" to "schemaCacheLoader".
This makes more sense since what we really want is the schema cache,
the version is the pre-requisite for ensuring our
schema cache queries work.

Additionally, we no longer log ` Attempting to connect to the database...`
at startup unnecessarily. This is only logged whenever there's a retry attempt.

test/io/test_io.py

@@ -778,7 +778,7 @@ def test_metrics_include_schema_cache_fails(defaultenv, metapostgrest):
                 r'pgrst_schema_cache_loads_total{status="FAIL"} (\d+)', response.text
             ).group(1)
         )
-        assert metrics > 3.0
+        assert metrics == 1.0
 
     reset_statement_timeout(metapostgrest, role)