common/admin_socket: pass Formatter from generic infrastructure

common/admin_socket: pass Formatter from generic infrastructure

The implementation can choose to either use the provided Formatter, or
put something directly into outbl.  The implementation may choose to
flush the formatter to the output buffer|stream, or let the caller do it
for them (usually the latter).

Lots of fiddling/cleanup in the implementations to make this build,
including dropping the (seeminlyg unused?) ostream& output mode for
the librbd asok implementations.

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