Process multiple entities with a single queue item

I've run this on a site with 10,000 nodes. Here's the performance results with and without the patch.

With the patch - 500 entities per queue item

time vendor/bin/drush ssg -v --uri http://drupal8alt.test
>  [notice] 21 out of 21 total items have been processed. queries: 20278, memory: 380.5 MB, non-peak mem: 378.5 MB

________________________________________________________
Executed in    7.93 secs   fish           external
   usr time    4.93 secs  100.00 micros    4.93 secs
   sys time    0.52 secs  509.00 micros    0.52 secs

With the patch - 50 entities per queue item

time vendor/bin/drush ssg -v --uri http://drupal8alt.test
>  [notice] 201 out of 201 total items have been processed. queries: 21719, memory: 384.5 MB, non-peak mem: 384.5 MB

________________________________________________________
Executed in   10.18 secs   fish           external
   usr time    5.87 secs  138.00 micros    5.87 secs
   sys time    0.61 secs  650.00 micros    0.61 secs

Without the patch

time vendor/bin/drush ssg -v --uri http://drupal8alt.test
>  [notice] 5019 out of 10001 total items have been processed. queries: 50297, memory: 318.5 MB, non-peak mem: 316.5 MB
>  [notice] 9766 out of 10001 total items have been processed. queries: 47482, memory: 338.5 MB, non-peak mem: 336.5 MB
>  [notice] 10001 out of 10001 total items have been processed. queries: 2367, memory: 338.5 MB, non-peak mem: 326.5 MB

________________________________________________________
Executed in   21.25 secs   fish           external
   usr time   10.90 secs  115.00 micros   10.90 secs
   sys time    1.23 secs  650.00 micros    1.23 secs

The changes look good to me. I ummed and ahhed on queue vs dataset... if you like entities_per_queue_item then that's great! Thanks for the merge :)