[SPARK-3936] Add aggregateMessages, which supersedes mapReduceTriplets

aggregateMessages enables neighborhood computation similarly to mapReduceTriplets, but it introduces two API improvements:

1. Messages are sent using an imperative interface based on EdgeContext rather than by returning an iterator of messages.

2. Rather than attempting bytecode inspection, the required triplet fields must be explicitly specified by the user by passing a TripletFields object. This fixes SPARK-3936.

Additionally, this PR includes the following optimizations for aggregateMessages and EdgePartition:

1. EdgePartition now stores local vertex ids instead of global ids. This avoids hash lookups when looking up vertex attributes and aggregating messages.

2. Internal iterators in aggregateMessages are inlined into a while loop.

In total, these optimizations were tested to provide a 37% speedup on PageRank (uk-2007-05 graph, 10 iterations, 16 r3.2xlarge machines, sped up from 513 s to 322 s).

Subsumes apache/spark#2815. Also fixes SPARK-4173.
I don't understand - aggregateMessages needs the user to specify the triplet fields information; it can't reliably infer this on its own. Similarly, the user has to specify sendMsg.