osdc/Journaler: use pointers for out params

Fixes a style violation in the new JournalStream class.

Signed-off-by: John Spray <john.spray@inktank.com>
doc: Update ceph-mds manpage

Add missing --hot-standby and --journal-check
arguments.

Fixes: #8258
osdc/JournalStream: DRY envelope size calc

Replace repetitive sizeof() calculation for the
envelope overhead per log event with #defines.

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