Instead, decline the session and do a full handshake. The semantics of
cross-version resume are unclear, and all major client implementations
treat this as a fatal error. (This doesn't come up very much, mostly if
the client does the browser version fallback without sharding the
session cache.)

See BoringSSL's bdf5e72f50e25f0e45e825c156168766d8442dde and OpenSSL's
9e189b9dc10786c755919e6792e923c584c918a1.

Change-Id: I51ca95ac1691870dd0c148fd967739e2d4f58824
Reviewed-on: https://go-review.googlesource.com/21152Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>