Add a dead letter queue (DQL) implementation

[astubbs]

Implemented as a draft extention of termainl retry handling:

astubbs#8

Extension of:

• Let user function throw retry exception, instead of raw errors which pollute log #242

Related:

• Provide option for max retires, and a call back when reached (potential DLQ) #196

Workaround

Implement it yourself by producing to a dlq topic when you think you’ve retried too many times, and then don’t throw an exception so PC thinks it was successful