mgr: route the root-logger fallback through a module-independent sink

mgr: route the root-logger fallback through a module-independent sink

MgrRootHandler captures third-party logs from the Python root logger
and emits them through the _ceph_log method of whichever mgr module
last configured logging.  A handler on the root logger outlives any
single module, so once that module unloads the handler emits through
a stale pointer until another module reconfigures logging.

Add a free mgr_log() function to the ceph_module binding, emitting
via dout() like PyModuleRunner::log but with no PyModule involved,
and rewrite MgrRootHandler to route through it.  The handler no
longer references a module instance, so it cannot go stale.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>