Add API in BackupCursorService for new-style backup cursors

For this ticket:

1. Add a storage engine API for opening a backup cursor that returns a list of files that need to be copied.
2. Add a complementary method for closing.
3. Have the BackupCursorService use this open/close API.
4. Ensure BackupCursorService fails opening cursors when one is already open (regardless of whether it's via fsync or openBackupCursor).
5. Ensure BackupCursorService fails when using the wrong "close" method. I.e: using using fsyncUnlock after openBackupCursor must not work.

For later:

1. Adding the "metadata" document with replication information to the "open" result.
2. Adding the aggregation stage/exposing this to the client.