msgr: update Message envelope in encode, not write_message

Fill out the Message header, footer, and calculate CRCs during
encoding, not write_message().  This removes most modifications from
Pipe::write_message().

Signed-off-by: Sage Weil <sage@inktank.com>
msg/Pipe: prepare Message data for wire under pipe_lock

We cannot trust the Message bufferlists or other structures to be
stable without pipe_lock, as another Pipe may claim and modify the sent
list items while we are writing to the socket.

Related to #3678.

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