crypto/tls: use certificate cache in client

crypto/tls: use certificate cache in client

In verifyServerCertificate parse certificates using the global
certificate cache.

This should signficiantly reduce memory usage in TLS clients which make
concurrent connections which reuse certificates (anywhere in the chain)
since there will only ever be one copy of the certificate at once.

Fixes #46035

Change-Id: Icf5153d0ea3c14a0bdc8b26c794f21153bf95f85
Reviewed-on: https://go-review.googlesource.com/c/go/+/426455
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>