Reap singleUse actors

[matthew-white]

singleUse actors are for internal use:

• When a password reset is initiated, a singleUse actor is created and assigned the pwreset role. A 24-hour session is created for the actor, and the session's token is sent over email as part of a password reset URL.
• When a form is published, a singleUse actor is created and assigned the formview role in order to provide Enketo temporary access to the form. A 15-minute session is created for the actor.
• Formerly, Google Drive backups created singleUse actors. Google Drive backups have been removed.

In some cases, singleUse actors are deleted:

• If an actor with a pwreset assignment is used to change a password, the actor is then "consumed" (Actors.consume()).
• All singleUse actors created for Google Drive backups should be deleted now that that functionality has been removed.

However, I think that in other cases, singleUse actors are not deleted:

• 24 hours elapse before a password reset token is used. In that case, a new password reset must be requested. Yet the old singleUse actor is never deleted.
• Actors with a formview assignment are never deleted.

Looking at the QA server, I see:

• A smaller number of non-deleted actors with a pwreset assignment
• A much larger number of non-deleted actors with a formview assignment. A larger number makes sense given that every form will be associated with at least one formview actor. (It could be more than one if a request to Enketo fails.)
• I see a number of singleUse actors that are not deleted, yet do not have an assignment. In all cases, their displayName contains "Enketo sync token". I assume these correspond to deleted forms.
• Some deleted singleUse actors correspond to Google Drive backups. That's expected.

Given that these singleUse actors are tightly coupled with a single session, I think we should consider deleting them when we reap sessions (lib/task/reap-sessions.js). Specifically, if a session is expired, and the session was for a singleUse actor, then delete both the session and the actor.

We could also consider writing a migration to delete existing singleUse actors that are unneeded. To me, that seems lower-priority but still worthwhile.

[alxndrsn]

Is there any other record that a single-use actor has been created? If not, could they be important for audit purposes?