Handle fullRead callback after destroy

A user can request a sync, then immediately unsubscribe
In that case, the resource will be destroyed. In which case,
we should throw away any late replies from redis.
If the user wants to do this properly, let them wait for the sync
callback before unsubscribing.

/cc @zendesk/zendesk-radar


### Steps to merge
 - [ ] :+1: of the team

### References
 - Jira link: 

### Risks
 - None