Add support for findBySinceUpdate using since_update in metadata

[BrianNichols]

This functionality already exists:

Statement stmt = new Statement();
stmt.setNamespace(ns);
stmt.setSetName(set);

// Find records updated within the last hour.
QueryPolicy policy = new QueryPolicy();
policy.filterExp = Exp.build(Exp.lt(Exp.sinceUpdate(), Exp.val(60 * 60 * 1000)));

RecordSet rs = client.query(policy, stmt);

[qihaiyan]

thanks.