Draft: utxo-sync: Add basic file storage logic, and the ability to export a UTXO set to disk. `exportutxoset` RPC was added for testing

Depends on !1615.

Also added the ability to verify an exported UTXO set via RPC: verifyexportedutxoset.

This is more of a proof-of-concept, really. I wanted to push it just so that the work is saved somewhere public. Still a work in progress...