Multiple backups can be run for the same archive

In a customer case I saw that a customer was having multiple backup jobs being run on the same archive at once. cbbackupmgr creates lockfiles to prevent this but it appears that the operator always deletes them.

It's very important that only one cbbackupmgr runs against an archive at once and the program is built on that assumption. James Lee talked to Alex Emery about this previously but I think it got lost - the ideas then were:

1. Don't delete the lockfiles
2. Do, but have the operator maintain the invariant that only one backup per archive is run at once.