EnsuredOnce

The structures a store has made sure of (created, registered, format checked), by key: each is made sure of once, on its first use, and then costs the store nothing.

Concurrent first uses of one key wait for one ensure rather than each running its own, which would send its schema statements as many times at once. They wait on a ReentrantLock, not a monitor (computeIfAbsent): a virtual thread blocked on a monitor pins its carrier, and the ensure runs blocking driver calls. An ensure that fails keeps nothing; the next use runs it again.