Boston Mayor Martin J. Walsh and casino mogul Steve Wynn held a “helpful” meeting in Boston on Tuesday but did not reach an agreement over Wynn’s plans to build a $1.7 billion casino in the neighboring city of Everett. “It was a helpful meeting, a productive meeting,” said Michael Weaver, a spokesman for Wynn Resorts, which last year won a license to build the 24-story, curved-glass tower on the Mystic River waterfront.