Keep track of adhoc storage instances client jobs

* adhoc_storage: Add job client information

* adhoc_storage_manager: Now stores adhoc_storage_info records and
  provides functions to update them.

* adhoc_storage_info: New synchronized type keeping updated information
  about the client job assigned to a adhoc_storage instance

* adhoc_storage: The context can now be updated using a new update()
  function (non-synchronized for now, because it is only ever called
  via the adhoc_storage_manager).

* job_info: Type is now synchronized.

* errors: Add ADM_EADHOC_BUSY. Fix missing error messages for ADM_ENOENT
  and ADM_EEXISTS.

* abt/shared_mutex: Is now movable.