Simplified handling of already (or soon to be) mounted tapes.
Added support for next mounts.
Added logging of decision steps.

Removed the "--encryptionkey" option from "cta tape add"

The scheduling based on disk throughput estimates will not be implemented in the forseeable, if ever.

Added drive status reporting in scheduler
Added drive status reporting in OStoreDB
Added support for drive status listing in the front end
Removed virtual functions from the Scheduler, which is never overloaded.
Added DesiredDriveState structure to drive state.
Removed usage of duplicate MountType, DriveStatus and DriveState structures.
Created DriveInfo structure to allow recreation of drive register entry in all
reporting situation (potentially with partial/assumed info).

Ported DataTransferSessionTest unit test  DataTransferSessionGooddayMigration and DataTransferSessionEmptyOnVolReq.

This commit also enforces the interdiction of empty strings being
bound as string database parameters.  Either a non-empty string is
bound or no value at all is bound (nullopt).

Completed changes so that archive and retrieves can be queued, mounts created and jobs reported done.

Fixed archive queue not being deleted on last archive job pop.
Next step is retrieve mount scheduling.

Implementation of the top level retrieve request queueing in Scheduler.

Renamed drData (and its fields) to diskFileInfo to better reflect the use that we make of that structure

Removed deprecated structures from archiveNS and nameserver namespaces. Removed RetrieveToFileRequest.

Deleted tests for scheduler features that moved to the catalogue.
Factored out the common unit test initialization.

The values are now hardcoded.
Cleanup:
  - Removed object store unit tests for data now in the catalogue.
  - Removed now unnecessary 2nd step addition of retry limits to archive jobs.
  - Removed deprecated ArchiveToFileRequest files.