Make get_put_monitor_eqc pass during `make test`

Calling code:purge/1 and code:delete/1 were needed to make the test
pass. They effectively skirt using a sidejob worker to update folsom
stats, and do it directly instead. Since riak_kv is not running casts to
update folsom were being dropped on the floor that were needed by the
test. This only occurred in `make test` and not when running
get_put_monitor_eqc standalone because riak_kv was started and shutdown
before this test ran during `make test`.

Additionally, I added timeouts for the polling in case of a regression,
made the precondition more specific and ran this test in a slave node.
While these changes aren't strictly necessary they provide for cleaner
code and a better example for future tests.

I also removed some dead code.