Consider PathExpand direction in PathJoin

[BingqingLyu]

Is your feature request related to a problem? Please describe.

Currently, we support s-t path optimization, which split the s-t path query into two sub-path expansions, and then join and concat the sub-paths into a single one, for efficiency. For example, when concating the sub-paths, we assume that the sub path directions looks like Path[1,2,3] and Path[5,4,3] where 1 and 5 are src and dst vertices in the original path, and thus we concat it into [1,2,3,4,5].

However, we haven't consider the true direction in PathExpand. For example, we may actually try to concat Path[1,2,3] and Path[3,4,5], and currently implementation would generate unexpected results. Please fix this.