buffer: add append(stringstream&)

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Monitor: make all commands be careful about stdout vs stderr

Status messages go to stderr, success or failure
Output data goes to stdout
Keep them separated

Signed-off-by: Dan Mick <dan.mick@inktank.com>