NewAdmissionController creates a new admission controller that will allow `parallelism` units of execution through at one time.

NewCustomAdmissionController creates a new admission controller that will allow `parallelism` units of execution through at one time.

DefaultM is default value for M for CoDel - roughly it's timeout of the queue when load is high.

DefaultN is default value for N for CoDel - roughly it's timeout of the queue when load is low.

Ticket must be used to Release resource back to admission controller.

An AdmissionController limits concurrent requests in order to protect a service from trying to do too many things at once.