merge 2 flists

[muhamadazmy]

This is tricky because store information is based on hash ranges, so this will not work if the 2 flists uses different stores because rfs will not know which store to use if a blob hash matches 2 stores.

rfs will try all stores that matches but it also means there can be a performance hit for each blob it has to retrieve.

• The merge must respect order of merge (like overlay) which flist is laid on top another flist
• The files from top flist should hide the ones from lower level.
• The dir from top flists should "merge" recursively with lower dirs