crypto/tls: change Conn.handshakeStatus type to atomic.Bool

crypto/tls: change Conn.handshakeStatus type to atomic.Bool

Change the type of Conn.handshakeStatus from an atomically
accessed uint32 to an atomic.Bool. Change its name to
Conn.isHandshakeComplete to indicate it is a boolean value.
Eliminate the handshakeComplete() helper function, which checks
for equality with 1, in favor of the simpler
c.isHandshakeComplete.Load().

Change-Id: I084c83956fff266e2145847e8645372bef6ae9df
Reviewed-on: https://go-review.googlesource.com/c/go/+/422296
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>