WIP: Adds max retries to Job

[tallysmartins]

A few considerations here:

Consideration 1
I've considered two ways of incrementing claim_count, and I need feedback to know which one is more appropriate.

1. First one is just doing claim_count + 1
2. Second one is a method increment_claim_count which is binded by prepare_changes to increment claim_count when the changeset transaction is completed.

From my point of view, first approach works and do not seems to present too much trouble considering possible concurrency between the runners queries.

Consideration 2
Do we need this configuration to be in the environment or something or are you ok with a module attr @max_retries 3?