Operation flags are not consistent

In some cases we use flags to signal behaviour, and in other cases we use explicit fields to signal behaviour. For instance in subdoc, the document-level flags are as explicit fields (mutatein_operation::access_deleted), but the spec-level flags are bitwise flags (mutatein_operation::entry::flags::xattr).

Additionally, it's a bit strange that the subdoc_opcode is a shared structure between lookupin and mutatein operations, but the flags for those specs is not shared.