20680: Set concurrency to num_cpus * 2 with a minimum of 8

20680: Set concurrency to num_cpus * 2 with a minimum of 8

In practice it makes sense to have two workers per core because
workers usually spend at least half their time waiting on the
database.

Setting a minimum concurrency of 8 avoids a deadlock situation when
controller calls keep-web, which calls back to controller.

This doesn't completely eliminate the problem, but it at least avoids
having deadlocked requests with a single user in the default
configuration (!).

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>