Version Control Cache

A redesign of the cache is needed for several reasons, among which the major ones are:

1. need to adapt to MultipleRepositorySupport.
   Since multiple repositories can be accessed in a given environment, each of them should have the possibility to be supported by a cache.
2. need to adapt to different vc backends.
   Now that Trac becomes more and more used with VCS other than Subversion, each backend should be able to use its own special kind of cache. A reasonably common base class (or set of base classes) should be nevertheless provided.
3. need to adapt to non-linear changeset history backends.
   The vc cache must be able to adapt to backends where each changeset can have multiple parents and children changesets.