messages: populate header.version in constructor

messages: populate header.version in constructor

Define a HEAD_VERSION and COMPAT_VERSION for any versioned message.  Pass
to Message constructor so that it is always initialized, even from the
the default constructor.  That's needed because we use that to check
decoding compatibility when receiving/decoding messages.

If we are conditionally encoding an old version, explicitly set
header.version in encode_payload().

We also set compat_version to demonstrate what will happen for future
revisions.  In this case, it's moot, because no old code understands
compat_version yet: nobody with old decode code will see these values
anyway.  But use this opportunity to demonstrate how it would be used in
the future.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>