Assign shrinking to one worker

[0xalpharush]

Right now all workers shrink a single failure and block fuzzing. We should pin the job to one worker (maybe all should be done if stopOnFailedTest is true but I don't think we're taking the min. of the workers or syncing them rn anyway)

see screenshot #448

[anishnaik]

The reason why each worker was shrinking in the screenshot is because the bug was easy to find so each worker began shrinking.

There is, however, validity to creating a "shrinking goroutine" or a pool of them that are responsible only for shrinking. There are drawbacks and benefits of this approach.