consumer: trigger backoff via E_FIN_FAILED

[jehiah]

A consumer can become overwhelmed with messages such that when it's completing them they have already server-side timed out. In this case the client thinks it's ok and keeps FIN'ing messages and getting more and may never be able to work off it's backlog (this situation is worse the higher max-in-flight used).

go-nsq could use the presense of E_FIN_FAILED to trigger backoff just like failing processing in the handler would. This will trigger RDY changes such that message flow drops off so the consumer can recover.

[mreiferson]

this is a great idea 👍