bin/petmail: use activate_this.py
source reorg: mailboxes have separate modules for delivery and retrieval
add CLI test for list-addressbook
add-mailbox: CLI and API, smoketest. No real functionality yet.
update docs: rerandomized ElGamal-encrypted recipient-ids

This should work to prevent senders from linking the recipient-id
portion of the transport descriptors as belonging to the same
recipient (reducing the anonymity set down to the number of recipients
sharing the same mailbox server).
might finally have a correct protocol
added rerandomizable tokens (with stubbed-out crypto)
start updating mailbox.rst with the latest protocol
add a few crypto concerns to investigate later
more protocol/docs updates