Add timer:sleep()-based throttle to riak_kv_exchange_fsm:read_repair_keydiff()

The throttle part is easy: a 1-liner.  The rest is the infrastructure
to poll each node for its maximum vnode mailbox queue length, pick the
worst one, calculate a sleep time based on a list of {Q_length, SleepMSec},
and communicate via time-honored application env vars.

The only hot code path that reads the ETS table for app env vars is
riak_kv_exchange_fsm:read_repair_keydiff() itself, and since it is
doing a full KV get FSM plus other stuff, the extra heat on the hot
path is insignificant.
Fix non-eunit compilation error
Add kill switch
Use async worker proc to query vnode status for AAE throttle check
Merge pull request #754 from basho/slf-aae-throttle

Add timer:sleep()-based throttle to riak_kv_exchange_fsm:read_repair_keydiff()