This should have been 2 bytes all along, since version numbers can
be 16 bits long. This isn't a live bug, since the call to
is_or_protocol_version_known in channel_tls_process_versions_cell
will reject any version number not in the range 1..4. Still, let's
fix this before we accidentally start supporting version 256.
Reported pseudonymously. Fixes bug 8062; bugfix on 0.2.0.10-alpha --
specifically, on commit 6fcda529, where during development I
increased the width of a version to 16 bits without changing the
type of link_proto.
