crypto/tls: pool Conn's outBuf to reduce memory cost of idle connections

Derived from CL 263277, which includes benchmarks.

Fixes #42035

Co-authored-by: Filippo Valsorda <filippo@golang.org>
Change-Id: I5f28673f95d4568b7d13dbc20e9d4b48d481a93d
Reviewed-on: https://go-review.googlesource.com/c/go/+/267957


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Roberto Clapis <roberto@golang.org>