msg/Pipe: fix msg leak in requeue_sent()

The sent list owns a reference to each message.

Signed-off-by: Sage Weil <sage@inktank.com>
msg/Pipe: associate sending msgs to con inside lock

Associate a sending message with the connection inside the pipe_lock.
This way if a racing thread tries to steal these messages it will
be sure to reset the con point *after* we do such that it the con
pointer is valid in encode_payload() (and later).

This may be part of #3678.

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