Disable background GC by default; bump credit flow defaults

Quite a bit of evidence of different kinds of workloads suggest
that background GC does not contribute meaningfully to reducing
node RAM usage on most of them. It does, however, on each run
produce a massive spike in VM and CPU context switches,
causing latency spikes.

Some users report that running without background GC cuts
their latency to a half or even a third.

Multi-hour stress tests that max out all node CPU cores
suggest that our current credit_flow default can still be bumped
safely, reducing the amount of time publishers are throttled
and making publisher latency and node's observed message throughput
more predictable.

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