msg: add alternate statuses for ms_dispatch2 handling

msg: add alternate statuses for ms_dispatch2 handling

Many dispatchers return false to allow other dispatchers also common messages
like MOSDMap or MFSMap. They implicitly depend on some dispatcher which is
always at the "tail" of the dispatcher queue to return "true" indicating the
msg was processed to avoid messages like:

    2025-02-18T05:31:17.738+0000 7f5206546640  0 ms_deliver_dispatch: unhandled message 0x5632d05f0700 fsmap(e 9) from mon.0 v2:172.21.3.230:40412/0

but this cannot always happen when some libraries like the RadosClient used standalone.

So, add a variant for encapsulating other indications for how the message was
processed by dispatch2.  For example, a message may be "acknowledged" but
explicitly allow other dispatchers to try processing the message.

Note: we're using a variant to avoid updating all of the ms_dispatch code to
use the sentinel classes.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>