Can execute basic queries over request-response.

Change interface AvaticaPrepareResult to class Meta.Signature.

Move DateTimeUtil to Avatica and rename to DateTimeUtils.

Linq4j.iterableEnumerator checks whether the Iterable is already an Enumerable; IterableEnumerator calls close() if the Iterator implements Closeable.

Add test case for [CALCITE-508] (not fixed yet).