branchcache: drop the unused `_verifyclosed` This code appears dead since its introduction about 5 years ago in this three consecutive commits: - 6578654916ae → introduce the method with two calls - 7c9d4cf23adf → remove first call - be5eeaf5c24a → remove second call o changeset: be5eeaf5c24a | user: Pulkit Goyal <pulkit@yandex-team.ru> | date: Fri Apr 05 15:57:09 2019 +0300 | summary: branchcache: don't verify closed nodes in _branchtip() | o changeset: 7c9d4cf23adf | user: Pulkit Goyal <pulkit@yandex-team.ru> | date: Fri Apr 05 15:56:33 2019 +0300 | summary: branchcache: don't verify closed nodes in iteropen() | o changeset: 6578654916ae | user: Pulkit Goyal <pulkit@yandex-team.ru> ~ date: Mon Apr 01 13:56:47 2019 +0300 summary: branchcache: lazily validate nodes from the branchmap

branchcache: dispatch the code into the dedicated subclass The code useful only to the local brancache have now been moved into the dedicated subclass. This will help improving the branchcache code without subtle breaking the remote variants.

branchcache: introduce a base class for branchmap This will help define a clear boundary between the two.