Repair should move aside unsalvageable data files and create empty ones in their place

If a collection is unsalvageable by WiredTiger, the data file should be moved aside (potentially to a .corrupt file name), and a new, empty collection should be created in its place.

Currently, repair fails if a collection is unsalvageable. This behavior should be changed so that the database can be started normally even if collections cannot be salvaged.