GODRIVER-3108 Optimize writeServerSelector

[isopov]

Summary

Optimize allocations of Server slices during writes.

Background & Motivation

On one of my services this function allocates 295 GBs over several days.

This is very similar to #1107 #1108 #1111. NB: one of these optimizations was reverted for correctness - 26e6c8a I used similar here, but cannot judge wether it may be used here.

[mongodb-drivers-pr-bot]

API Change Report

No changes found!

[prestonvasquez]

Hey @isopov , thanks for the contribution. In addition to the suggested changes, could you add the following benchmark to mongo/description/selector_test.go:

func Benchmark_SelectServer_SelectServer(b *testing.B) {
	topology := Topology{Kind: ReplicaSet} // You can change the topology as needed
	candidates := []Server{
		{Kind: Mongos},
		{Kind: RSPrimary},
		{Kind: Standalone},
	}

	selector := writeServerSelector{} // Assuming this is the receiver type

	b.ReportAllocs()
	b.ResetTimer()

	for i := 0; i < b.N; i++ {
		_, err := selector.SelectServer(topology, candidates)
		if err != nil {
			b.Fatalf("Error selecting server: %v", err)
		}
	}
}