clients still may need a handler pool to spawn lookup ULTs (in
the case they are attaching to a group)

added margo functions and macro to register RPCs and avoid using a global mapping between hg_context and margo instances

- yield was in wrong if block, not sure how

- count ULTs in progress pool that are waiting on communication as
  waiters so that we don't busy spin when it isn't necessary
- if a trigger occurs, yield immediately to service ULTs that may have
  been activated as a result
- HG_Progress() is still called too frequently, see TODO note
- outcome: this makes servers significantly faster, but seems to make
  clients a bit slower; optimal performance right now comes from
  building servers with this git commit and building clients without
  it; need to investigate