Remove mock-based system tests, rename "unit" to "wire".

The mock-based tests tested a strict subset of what system-fullstack
does, and were generally harder to work with, so there's no reason to
keep them around.
Fix join tree scanning breaking on packet loss or latency spike.
Reduce number of ht scan processes to 8.
Don't try to make commits before the local node has an id.
Add grace period to commit lag checks.
Fix using wrong clock to generate DENY chmods.
Add bloom filter mechanism to HtRangeNext.

Allows clients to detect packet loss. Unfortunately, this still isn't
enough to handle even 12% packet loss between nodes, due to interactions
with the visibility threshold, the htranges, and Received messages.

Essentially, we need yet another packet-loss-recovery mechanism.
Define and stub out indermediate packet loss recovery mechanism.
Implement linear ack mechanism.

Probably still needs some more tuning. Before it's fully usable, we need
to make system know how to wait until positive-disposition nodes fulfil
their commitments.
Add join state to wait for commit/validation stability.
Fix off-by-one on Ack messages.
Fix doubling estimated latency into clock source.
Increase default max_{commit,validated}_lag.
Add test around operation on lossy, latent network.