MB-11873: fixed logging old_node_marker vbopaque

xdcr trace infrastructure needs it to be ejson. And old marker wasn't.

I've also updated few places where vbopaque was logged with assumption
that it's string.

Change-Id: I1d8cbcb8e709884f135b45c7b01345adb691498c
Reviewed-on: http://review.couchbase.org/40272
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>