Handoff manager EQC testing

Starting with code written by @jrwest long ago, @aberghage and I have created a more comprehensive test suite using `eqc_statem` for `riak_core_handoff_manager`.

Tests include:

* Add outbound and inbound transfers
* Disable future transfers using `riak_core` environment variables
* Control concurrency level using `set_concurrency`
* Retrieve concurrency level using `get_concurrency`

Near-term plans for extending this:

* Test transfer lifecycle (errors at start or during transfer) as part of Alex's improvements to error status management #630 
* Test the more sophisticated enabling/disabling transfers as part of Alex's branch `feature/atb/handoff-enable-disable`
* Test resize transfers

current_concurrency() will never return a negative. We should be precise in our expectations. This occurs elswhere in the test.