exception reason should be escaped as well

* Elixir exception resons aren't a simple atom like the Erlang ones.
PIDTable keeps a reference count

* Every function call would be wrapped in PIDTable methods
  start and finish, so in order to kill a Coordinator, we need to
  count the number of user processes of a Coordinator.
context changing on function calls

* using PIDTable.start/finish manage the ctxt on the Coordintator.
* is PIDTable a good name for a process handling function calls &
  context changes?