Fixed spurious file closing on failure to open in BackendVFS.

Fixed race condition detected by helgrind in ArchiveMount.

Agent becomes a usual class, like any other in the object store.
Object name generation (agent's and others) is now handled in the separate AgentReference.
The previous situation created race conditions.

Ported DataTransferSessionTest unit test DataTransferSessionMissingFilesMigration and DataTransferSessionTapeFullMigration.

Also fixed tapes not being set to full when reaching end of tape.

Ported DataTransferSessionTest unit test  DataTransferSessionGooddayMigration and DataTransferSessionEmptyOnVolReq.

Ported another DataTransferSessions unit test (DataTransferSessionWrongRecall) to the new scheduler.