[CALCITE-93] Calcite RPC server;
[CALCITE-94] Remote JDBC driver

Add module, calcite-avatica-server. HttpServer is an RPC server that listens on port 8765 for JSON requests over HTTP.

We can now execute basic queries, and schema and catalogs metadata requests, 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).

For remote testing, allow CalciteAssert to test all connections, not just CalciteConnection.