gossip: Don't check if the network should be tightened so often

[a-robinson]

trafficstars

Touches #51838. See the thread there for more detail, but the gist is that tightenNetwork gets called hundreds of times per second (taking a full mutex lock each time) on large clusters, even when nothing needs tightening.

Release note (performance improvement): Avoid wasteful contention on the gossip mutex caused by checking if the network needs tightening hundreds of times per second.

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[cockroach-teamcity]

This change is 

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[a-robinson]

The release note isn't needed for this change because it doesn't require documentation changes.

Does that mean you'd like me to remove it from the commit?

[craig[bot]]

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