cbimport may hang after receiving a max retries error

What's the issue?
Prior to 6.6.0, if 'cbimport' received a "fatal" error from the restore pool it would continue processing other documents and just print a warning. Upon receiving a fatal error, the worker pool will teardown (this is the desired behavior for 'cbbackupmgr'). This means that when 'cbimport' attempts to continue importing data, it hangs.

Is this going to happen frequently?
No, it shouldn't it will should only happen in serious failure scenarios e.g. max retries reached.

What's the fix?
We need to stop the restore pool for tearing down upon receiving a failure, however, we must still maintain the ability to gracefully shutdown so that 'cbbackupmgr' can teardown the worker pool after receiving a "fatal" error.