fix #929: wait for riak_kv_stat to unregister

The {{already_started, ...}, {child, ...}} error noted in the description
of issue #929 is caused by an instance of riak_kv_stat trying to register
its name when another instance is already registered. Add
riak_kv_test_util:wait_for_unregister/1 and use it in
riak_kv_test_util:cleanup/3 to wait for the registered instance of
riak_kv_stat to unregister and stop. Remove similar functionality in
test/get_put_monitor_eqc.erl and make that module call
riak_kv_test_util:wait_for_unregister/1 instead.

Also add goldrush to the app list in riak_kv_test_util:dep_apps/2 to ensure
it's started and stopped correctly; this prevents crash dumps caused by
{application_terminated,goldrush,killed} at the end of "make test".
Use binary_to_atom/2 instead of binary_to_existing_atom/2.

There's a possibility that the riak_dt code that contains the set
operation atoms hasn't been loaded when
riak_kv_crdt_json:set_op_from_json/1 is invoked, meaning
binary_to_existing_atom/2 will raise a badarg. We are already checking
the incoming operation as a binary via a guard on the relevant
function clause, so it is safe to use binary_to_atom/2 instead.
Whitespace cleanup
Erlang environment changes for R16B02-basho5
removed TODO saying to fix the thing I just fixed
Merge pull request #935 from basho/bugfix/jnd/everybody-to-the-limit

Fix limit checks in riak_kv_env for R16B02-basho5
Merge pull request #933 from basho/bugfix/sdc/http-crdt-badarg

Use binary_to_atom/2 instead of binary_to_existing_atom/2.
Merge pull request #932 from basho/vinoski/test-fixes

fix #929: wait for riak_kv_stat to unregister
fix #937: spawn tests to avoid unrelated messages

On slow systems the riak_kv_eleveldb_backend retry tests could fail due to
gen_event timeout messages from previous activities. Use a test fixture to
spawn these tests to ensure clean message queues.
fix #936: handle slow metrics for get_put_monitor_eqc

On slow systems the get_put_monitor_eqc test could fail due to not waiting
long enough for folsom metrics updates. Add bounded wait for metrics to
become non-negative before checking them against expected results. Also
increase max test time from 120 to 150 to avoid test timeout on slow
systems.
Merge pull request #938 from basho/vinoski/fix-936

fix #936: handle slow metrics for get_put_monitor_eqc
Merge pull request #939 from basho/vinoski/fix-937

fix #937: spawn tests to avoid unrelated messages

Reviewed-by: reiddraper