Disconnect and cancelled connection tests

This expands on the existing connection manager tests adding a disconnect_test() and an error_cleanup_test(). The disconnect test is fairly simple and mirrors the simple_test() with the addition of verifying correct disconnect procedure. The error_cleanup_test() is meant to check that cancelled connections are cleared from the riak_core_cluster_mgr state after a configurable interval(cm_cancellation_interval).  A failure of the error_cleanup_test() can be shown by setting `cm_cancellation_interval` to `never`.

This test is meant to exercise the logic in basho/riak_repl#598