cubicweb #2151672 Have bfss create read-only files [validation pending]

The bfss never alter a files it create (it always replace it with another one). Creating read-only files will ensure alterationf of bfss file will never happen by mistake. This would be usefull to allow sharing (with hardlink) huge file base between production and preproduction while being sure no alteration happen.

done in3.15.0
load left0.000
closed by#1f6ba9afb925 [storage] BFSS now create read only file (closes #2151672)