Allow cache-write-only to overwrite existing cache entries

[bigdaz]

There is now an API that permits users to delete specific cache entries. It might be possible to leverage this to overwrite existing cache entries, allowing corrupted cache entries to be removed.

Two things we could do without user input:

• When running with cache-write-only, we could automatically delete any existing cache entries. This would have the effect of overwriting existing cache entries with the same key.
• If a cache entry is obviously corrupted (ie cannot be restored) we could delete it.

[bigdaz]

This would address #20