messages/: always set header.version in encode_payload()

messages/: always set header.version in encode_payload()

we encode the payload w/o the writelock even can_write == NOWRITE, if
the message "can_fast_prepare". in that case, the "feature" of the
connection is 0, as no handshake happens yet. so the header.version is
always set to a version compatible with pre-luminous. but when the
message is re-encoded when the connection is re-established with feature
with luminous, the header.version is not set back to HEADER_VERSION.
that's why the message's encoding is not consistent with header.version
sometimes.

in this change, we always set the header.version in encode_payload(), so
it's consistent even after connection reset and message re-encoding.

Fixes: http://tracker.ceph.com/issues/19939
Signed-off-by: Kefu Chai <kchai@redhat.com>