1. A storage management system for scalable auxiliary-copy processing using media agent resources, the storage management system comprising: a storage manager for reserving one or more data streams for transfer of data in a first auxiliary-copy job from a source to a destination in the storage management system, wherein the source comprises a first copy of data generated in the storage management system, and further wherein the destination stores a second copy of the first copy, which second copy of the first copy is generated during the first auxiliary-copy job and is designated an auxiliary copy; a first media agent in communication with the storage manager; a second media agent in communication with the storage manager and with the first media agent; wherein the first media agent is configured to: request, from the storage manager, data stream reservations for the first auxiliary-copy job, and coordinate with the second media agent the execution of the first auxiliary-copy job via the one or more data streams reserved by the storage manager; and wherein the second media agent is configured to: fetch, from the storage manager, job-specific metadata about the first auxiliary-copy job, generate the auxiliary copy, based at least in part on the job-specific metadata fetched from the storage manager.