changefeedccl: don't busy-wait on schema_locked in schema feed

Currently, schema_locked allows us to not poll for descriptor updates. However, we're still busy-waiting on a mutex-guarded bool to check if schema_locked is still true. We could maybe replace this with a channel or some other kind of signaling mechanism (e.g. condition variable).

https://github.com/cockroachdb/cockroach/blob/f701074bdb42dbd7e078b3ce8a0ee4cbaca82fab/pkg/ccl/changefeedccl/schemafeed/schema_feed.go#L321

Jira issue: CRDB-49397