crypto/tls: convert Conn.activeCall to atomic type

crypto/tls: convert Conn.activeCall to atomic type

Change-Id: I5b063070a17bdeed57e73bfb76125b94268b3bc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/426088
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>