Implement basic orphan detection

[efoxepstein]

We need EDPs and MPs to periodically call /ExchangeStepAttempts.ListExchangeStepAttempts to look for orphaned ESAs.

One way to do this:

1. Client writes a lock file somewhere locally when it starts working on an ESA (e.g. using the Storage interface to write to private storage).
2. Client calls /ESA.ListESAs with a filter to exclude recently created things and checks for a lock file on each. If there isn't a lock file, it should just start executing the ESA or mark the attempt as failed. If there is a lock file, it should check the age of it -- we might want to alert on this.