Make get_put_monitor_eqc pass deterministically

[andrewjstone]

• Change check_state to be the optional invariant callback.
• Just return true from postcondition since it was only used to check
  the state.
• remove unneccessary timer:sleep/1 calls
• Poll for folsom stat changes instead of relying on timer:sleep. This will fix
  the test on slow machines such as the builders. It also makes the test
  run a lot faster as it no longer sleeps arbitrarily.
• Move test initialization out of the initial_state callback
• Log the metric that failed for invariants
• Poll active stats during get and put fsm shutdown/exit since the
  count needs to be decremented
• swap ordering of monitoring the pid and sending it a kill msg in
  end_and_wait/2

[jrwest]

Huge improvement but not 100% there yet. Still seeing a regular failure due to a race when running 10k iterations. the test passes using eqc:check every time afterwards.

Failed Metric = {riak_kv,node,gets,fsm,active}
QC result: {invariant,
               {'EXIT',
                   {{assertEqual_failed,
                        [{module,get_put_monitor_eqc},
                         {line,163},
                         {expression,"Val"},
                         {expected,3},
                         {value,4}]},
                    [{get_put_monitor_eqc,'-invariant/1-fun-0-',2,
                         [{file,"test/get_put_monitor_eqc.erl"},{line,163}]},
                     {get_put_monitor_eqc,'-invariant/1-lc$^0/1-0-',1,
                         [{file,"test/get_put_monitor_eqc.erl"},{line,163}]},
                     {get_put_monitor_eqc,invariant,1,
                         [{file,"test/get_put_monitor_eqc.erl"},{line,157}]},
                     {eqc_statem,run_commands,2,
                         [{file,"../src/eqc_statem.erl"},{line,704}]},
                     {get_put_monitor_eqc,'-prop/0-fun-0-',1,
                         [{file,"test/get_put_monitor_eqc.erl"},
                          {line,50}]}]}}}
Shrinking.(1 times)
[{set,{var,1},{call,get_put_monitor_eqc,get_fsm_started,[]}},
 {set,{var,2},{call,get_put_monitor_eqc,put_fsm_started,[]}},
 {set,{var,3},{call,get_put_monitor_eqc,put_fsm_noproc,[]}},
 {set,{var,4},{call,get_put_monitor_eqc,put_fsm_exit_normal,[put,{var,2}]}},
 {set,{var,5},{call,get_put_monitor_eqc,get_fsm_started,[]}},
 {set,{var,6},{call,get_put_monitor_eqc,get_fsm_started,[]}},
 {set,{var,7},{call,get_put_monitor_eqc,get_fsm_noproc,[]}}]

[jrwest]

its good to do clean up after the iteration runs, also. That way the last iteration doesn't leave around artifacts that mess with tests that run later in the suite.

[andrewjstone]

I used eqc_statem:pretty_commands/4 instead of ?WHENFAIL

[jrwest]

+1 merge. 110k iterations passed. nice work.