crypto/tls: use SessionState on the client side

Another internal change, that allows exposing the new APIs easily in
following CLs.

For #60105

Change-Id: I9c61b9f6e9d29af633f952444f514bcbbe82fe4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/496819
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>

cache.go

@@ -39,7 +39,7 @@ type certCache struct {
 	sync.Map
 }
 
-var clientCertCache = new(certCache)
+var globalCertCache = new(certCache)
 
 // activeCert is a handle to a certificate held in the cache. Once there are
 // no alive activeCerts for a given certificate, the certificate is removed