Parallelize key creation in chef_keygen_cache

This patch introduces a new app config `{chef_authn,
keygen_cache_workers, WorkerCount}` that controls the number of
concurrent keygen workers that will be used to fill the cache to its
max level.

The value returned when a worker times out changed from 'timeout' to
'keygen_timeout' to distinguish it from gen_server system timeouts.

The default worker count is set to half of the number of logical
processors available on the system.