* VerificationQueue don't spawn up extra threads
In the verification queue we spawn up worker threads to do the work.
However, if `num-verifiers` is specified we still spawn the maximum
number of threads which consume extra memory.
There is one catch though when `--scale-verifiers` is specified then
we can't do it because all threads are created upon initilization AFAIK.
In my opinion, is doesn't to use both `num-verifiers` and
`scale-verifiers` they are kind of contradictory!
* Fix nits in logic and add tests for verification
* refactor(verification queue) - rm hardcoded const
* Address grumbles in new tests
* Remove hardcoded `MAX_VERIFIERS` constant and replace it by relying
entirely on `num_cpu` crate instead inorder to support CPUs that have
more cores/logical cores
3216b143c2