crypto/tls: reduce the number of allocations made for Conn.Read calls

[TriAnMan]

Conversion of atLeastReader to a field for Conn structure prevents atLeastReader from being allocated on each call of Conn.Read

Fixes #53142

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This PR is being closed because golang.org/cl/409334 has been abandoned.

This CL (imported from a PR) is targeting the "release-branch.go1.18" branch. Go 1.18 is no longer supported per the release policy and so we don't use the release-branch.go1.18 branch for anything now.

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