concurrent reads on ReadTopologyInstanceBufferable

[shlomi-noach]

Some reads on ReadTopologyInstanceBufferable() are safe to run concurrently. Such are reads that subsequent actions do not rely upon.

Running such reads concurrently reduces instance read latency.

[shlomi-noach]

On some remote DC, high latency network, this reduced instance detection time from ~560ms to ~250ms. This is a substantial reduction.

For local DC or low latency networks I see little difference (and this is expected).

[shlomi-noach]

1cb9736 further reduces to ~190ms

[shlomi-noach]

This deploys well.