osd: data loss: pg export/import/remove

In progress: New version of export
Old import code was squashed here

feature #4201

Signed-off-by: David Zafman <david.zafman@inktank.com>
Add template function write_section()
Add write_simple() for ending types
Separate header from omap_section
Add initial part of import
   Get super header
   Get pgid
Skeleton of import
Temporary maximum read length 1024
Fix export data offset bug
Data import works
Import attributes
Add omap header
Pass transaction and apply it for all object changes
Add omap
Rename section for data to data_section
Add flag to force version write in _write_info()
Add struct_v to metadata_section
Add struct_v to write_info() and use new flag to force ver to be written
Remove old code