crypto/tls: replace custom intern cache with weak cache

crypto/tls: replace custom intern cache with weak cache

Uses the new weak package to replace the existing custom intern cache
with a map of weak.Pointers instead. This simplifies the cache, and
means we don't need to store a slice of handles on the Conn anymore.

Change-Id: I5c2bf6ef35fac4255e140e184f4e48574b34174c
Reviewed-on: https://go-review.googlesource.com/c/go/+/644176
TryBot-Bypass: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>