Add support for multi-label edge patterns.

[glennga]

Currently, we only support edge patterns with a single label. Again, it would be nice to support more complex edge patterns in queries. For now, users have to simulate multi-label edges by using composition in the edge definition body (given in the workaround below):

FROM
  GRAPH MyGraph
    (u1:User)-[:IS_FRIENDS_WITH|IS_BEST_FRIENDS_WITH]->(u2:User)
SELECT *;

must be written as:

WITH
  GRAPH MyTemporaryGraph AS
    VERTEX (:User)
      PRIMARY KEY (user_id)
      AS Users,
    EDGE (:User)-[:IS_FRIENDS_OR_BEST_FRIENDS_WITH]->(:User)
      SOURCE KEY (source_id)
      DESTINATION KEY (dest_id)
      AS (
        <IS_FRIENDS_WITH edge definition>
        UNION ALL
        <IS_BEST_FRIENDS_WITH edge definition>
      )
FROM
  GRAPH MyTemporaryGraph
    (u1:User)-[:IS_FRIENDS_OR_BEST_FRIENDS_WITH]->(u2:User)
SELECT *;