mds: introduce explicit DaemonState instead of int

Signed-off-by: John Spray <john.spray@redhat.com>
mds: handle replaying old format journals

To get back to the reformatting procedure that otherwise
occurs during MDLog::open, introduce an MDLog::reopen call
that MDS can use in the standbyreplay->standby transition
for the special case where the journal is old.

Fixes: #8869

Signed-off-by: John Spray <john.spray@redhat.com>
mds: remove some rogue "using namespace std;"

Signed-off-by: John Spray <john.spray@redhat.com>