Inconsistency between `getReplicasForJobs` and `RunningLimit`

[chaen]

getReplicasForJobs (in the API/DataManager) returns only replicas that are allowed for job, i.e. on disk, not on failover, etc. However, the RunningLimit of a given site can be set to 0 for User jobs, making the job impossible to run. We could detect that in the Dirac API and return an error.