Only one WT backup cursor can be open at a time. The fsync + lock command opens a backup cursor which is kept open until `fsyncUnlock` is called.

The new filesystem backup story will continue to use backup cursors, but via a different command (that among other things, does not take out a global exclusive lock). There will also be new storage engine API methods for working with the new filesystem backup story.

This ticket is to create a service class that sits above the storage engine that will dictate the semantic that only one cursor is allowed open at a time. Also part of this ticket is having `fsyncLock/Unlock` go through this service class.

New methods for the upcoming backup work should not be speculatively done as part of this ticket.