Assign Job Key ID when none supplied in produce_block_with_custom_id()

[alfiedotwtf]

Currently when no custom_id is supplied to produce_block_with_custom_id(), the job manager will create the job with no key. However if invoked again while the first job is still running, as the second None key already exists from the first job, it will error with JobAlreadyExists.

This fix will generate a random key if None was provided.

[alfiedotwtf]

We also need to allow for the following scenarios:

• calls to produce_block_with_custom_id() need to be allowed to run in parallel while testing
• calls to produce_block() need to be allowed to run in parallel while testing
• need to prevent the same staking key to be used in parallel because there could be penalties