flagprocessors: use _processflagswrite for write operation There are no ambiguity for 'write' operation so it is simple to replace. Differential Revision: https://phab.mercurial-scm.org/D6801

flagprocessors: introduce specialized functions This make the call site clearer and the open the way to more diverse return types. For now, the same old code is still in use under the hood. Differential Revision: https://phab.mercurial-scm.org/D6800

flagutil: use it in simplestorerepo This remove the other code duplication of the `_processflags` code. To be honest, this code looks very dead since it is not run by either developer nor buildbot. However, we update the code to make the task of reviving this less daunting. Differential Revision: https://phab.mercurial-scm.org/D6799

flagutil: make the error class used by the mixin configurable One of the code duplication use a different error class. So let's make it possible to do so. Differential Revision: https://phab.mercurial-scm.org/D6798

flagutil: use the new mixin use in remotefilelog This remove one of the code duplication. Hooray \o/. Differential Revision: https://phab.mercurial-scm.org/D6797

flagutil: introduce a flagprocessorsmixin class To avoid code duplication, we will provide a simple "ready to use" mixin that carry the appropriate logic. First we use it in standard revlog, we'll remove code duplication in later changesets. Differential Revision: https://phab.mercurial-scm.org/D6796