Implemented event rejection, which should resolve and prevent future netsplits of the kinds observed
within some Continuwuity rooms.
Also resolved several bugs related to both soft-failing events, and event backfilling, which should
improve state resolution stability.
The `!admin debug get-pdu` command was updated to disambiguate event acceptance status, and
`!admin debug show-auth-chain` was added to visually display event auth chains, which may assist
developers in debugging strangely complex events.

Contributed by @nex.
