circmgr: limit circuit attempts when launch_parallelism > 1.

Previously, if we had launch_parallelism > 1, and we were willing to
retry building a circuit max_retries times, then we'd launch up to
max_retries * launch_parallelism circuits before giving up.  Ouch!

With this patch, we try to keep the total number of circuits
planned and attempted to the actual max_retries limit.

Part of #329; found with arti-testing.