Define a "time to live" on proxied resources

[Adam Staveley]

Resources (URLs) are stored and proxied when they require authorization on a counterparty server that the recipient does not have. For example, the response_url in commands RECEIVER requests, or paginated Link response headers.

Currently, the OCN Node stores these proxied resources until used. We could, however, defined a TTL on these resources, such that are deleted unless they are “redeemed” by a certain period (e.g. 24 hours).