The Live WT restore functionality depends on having a background thread/s that migrates data across in the background.

At this stage the planned algorithm is:

1. For every file in WiredTiger.backup that is not metadata.
2. Find the extent lists for the file. (How?)
3. For every hole in the extents for that file, read from source and write to destination

There are a few problems that haven't been solved yet:

• Given WiredTiger owns the file handle how do we access it's extents?
• How do we deal with concurrent schema operations?
• Concurrent access to the extent list needs locking.
• How to scale threads reasonably?

Given that the problem space is still largely unexplored the scope of this ticket will be to:

1. On intialization of the Union FS create a WT thread group
2. In the thread run body find a method to access the file handles that are open in WT.