…gation

Use some concurrency by default, and increase the heartbeat interval.

Log when we're waiting for DNS propagation.

If there are many historical logs in the stream, you can get a lot of
output when reading from the beginning of time and the operator is
usually only interested in what's happening right now.

We were not properly tearning down these instances, which lead to
lingering open RocksDB instances and open file descriptors to removed
files, preventing disk from being reclaimed.

Simplify Router by starting a Channel immediately, and instead emphasize
RPC timeouts rather than connection timeouts.

There are a variety of conditions where connection timeouts don't really
work, such as when a server has bound its port but is not actively
serving it. Currently this causes tonic to block indefinitely.

Also, Channel embeds a reconnection behavior if the transport is broken,
so it's already the case that Channels are connecting in the background
and we should expect to see corresponding delays.

So, have a single behavior for Channels: they start immediately but may
have connection delay in the background.

- Back out connection-readiness route selection in favor of random balancing.
- Distinguish local vs non-local Channels: in the future, we should use this
  to implement selective compression over non-local Channels.

Also, upon an error a Read stream must clear its ReadRequest Header.
The rationale is that the current Header could point to a Gazette broker
which has permanently stopped, and connections will time out repeatedly.
Clearing the Header causes route discovery to restart using
the base service address.

Lead with an identity-transform example SELECT JSON($flow_document)

Continue to emit an example selecting individual fields, but
semantically disable it and also remove root document projections,
because these are never desired.

HTTP/2 keep-alive sends a PING frame every interval, and fails the
connection of the peer doesn't respond in time. This verifies the
end-to-end health of the HTTP/2 transport and catches issues like
servers which have bound sockets but aren't actively listening.

Also using HTTP/2 keep-alive when connecting to local containers. We've
observed that `podman` can fail in ways that leave the reactor believing
it has an established connection to flow-connector-init, even though the
container has failed and the network namespace has been torn down.