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>
osdc: make JournalStream::readable const

Signed-off-by: John Spray <john.spray@inktank.com>
osdc: Simplify JournalStream::read

It was doing an unnecessary series of splices() after
reading header, replace with a single pass through
the data and one splice at the end.

Signed-off-by: John Spray <john.spray@inktank.com>
osdc: new style encoding for Journal::Header

Switch to the ENCODE_START, DECODE_START macros
for Journal::Header.  As well as being nice generally,
this has the important side effect of making
journal headers written since JOURNAL_FORMAT_RESILIENT
unreadable to older MDSs, as they will fail their check
on Header.magic.

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