Chain&Bundle store for offline verification

[thobson88]

Currently dDID verification involves resolving a chain and retrieving a bundle (to verify the timestamp on the root DID). Bundles are cached by the IONVerifier but not stored persistently.

Since these are small datasets, and in many use cases the same chains will be verified repeatedly, it would make sense to save the resolved chains and bundles to disk.

This could be especially useful for mobile nodes as it would enable offline verification.