Modernize ConfigCBDAccessSvc

• use dedicated type for read-only/read-write property
• use string_views::to_string instead of equivalent free standing implementation
• use boost::optional as return value instead of passing an uninitialized T by reference as INOUT argument, and returning a bool
• call file() less often / less implicitly, and instead assign the return value to a variable, keeping it alive in the local scope (in preparation of (maybe) eventually adding a lock around it, so that the underlying file isn't closed while read operations are still 'in flight')