branchmap: add the tipnode (cache key) on the branchcache object Gathering data and cache key paves the way to a lot of simplification.

branchmap: store branchcache in a dedicated object Value and key of branchcache would benefit from being hold by the same object. Moreover some logic (update, write, validation) could be move on such object. The creation of this object is the first step toward this move. The result will clarify branchcache related code and hide most of the detail in the class itself. This encapsulation will greatly helps implementation of branchcache for filtered view of the repo.