fix(swingset): don't deduplicate inbound mailbox messages

[warner]

The mailbox device tracking the highest inbound message number and ack for
each peer, to de-duplicate repeated messages, so it could reduce the amount
of kernel activity. Each call to deliverInbound would return a boolean to
indicate whether the messages/ack were new, and thus the kernel needed to be
cycled.

However, the device was holding this tracking data in non-durable state, so
if/when the kernel was restarted, the state would be lost. A duplicate
message/ack arriving in the restarted process would trigger kernel activity
that would not have run in the original process. These extra cranks caused
diverge between validators when one of them was restarted, and the client
sent a duplicate message (such as the pre-emptive ack all clients send at
startup). The extra crank does not get very far, because vattp does its own
deduplication, so the divergence was only visible in the slog. But when #3442
is implemented, even a single extra crank will flag the validator as out of
consensus.

The fix is to remove the mailbox device's dedup code, and rely upon vattp for
this function. The test was also updated to match, and a new test (comparing
two parallel kernels, one restarted, one not) was added.

closes #3471

[warner]

@FUDCo I recommend reviewing the two commits separately, the first is just refactoring.

[FUDCo]

Fewer bugs in less code. What's not to like?