Replace RoutingTableMessage with pair

RoutingTableMessage was used to construct routing tables to enable
joining VertexRDDs with partitioned edges. It stored three elements: the
destination vertex ID, the source edge partition, and a byte specifying
the position in which the edge partition referenced the vertex to enable
join elimination.

However, this was incompatible with sort-based shuffle (SPARK-2045). It
was also slightly wasteful, because partition IDs are usually much
smaller than 2^32, though this was mitigated by a custom serializer that
used variable-length encoding.

This commit replaces RoutingTableMessage with a pair of (VertexId, Int)
where the Int encodes both the source partition ID (in the lower 30
bits) and the position (in the top 2 bits).

Author: Ankur Dave <ankurdave@gmail.com>

Closes #1553 from ankurdave/remove-RoutingTableMessage and squashes the following commits:

697e17b [Ankur Dave] Replace RoutingTableMessage with pair

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