Implement a file handle cache (#54)

* Implement a file handle cache

To prevent constantly opening and closing in calls to cat_file.
Also, fix up the vector read size code to work correctly

lint

* Try to fire and forget sync close

* No TTL yet but the tests pass

* Implement TTL, but not checked in a watch loop

* Background cache pruner

* Play around with better typing a bit

* Test to show closing is ok while reading