nomad: fix race in BlockedEvals
nomad: fix race in BlockedEvals

I assume the mutex was being released before sending on capacityChangeCh
to avoid blocking in the critical section, but:

1. This is race.
2. capacityChangeCh has a *huge* buffer (8096). If it's full things
   already seem Very Bad, and a little backpressure seems appropriate.

hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad