Container sharing

Implements the sharing of containers by use of container_session_key_shares encrypted to third party's public keys.

Includes an example application (chat3) which works by storing Alice's messages to Bob, and Bob reading from her container. The demo is not polished, and absolutely contains bugs, but it works for two people, and I am really tired.

Also includes changes to retrieve records after a certain timestamp for more efficient polling on containers.

Still needs container_session_key signature verification, but that should be done at the same time as memoizing decryption of container_session_keys (currently done once per container_record).