access: an epoch's root is never dropped

A REFUSALS naming the swarm's first change made the node receiving it
judge that change again in a document that isn't empty, refuse it as
"a change without dependencies that isn't the epoch's root", and drop
it with everything built on it: its whole document.

The first epoch's root is now the first change a document holds, as it
comes to an empty one or as it is in one; a change judged again on a
peer's word is never refused for having no dependencies; and
drop_taken refuses to drop an epoch's root, or anything that would
leave nothing kept, logging an error instead.

Review finding 1 (critical).

0 files changed, 0 insertions, 0 deletions