We move from a binary "dirty" flag to a three value "state": "clean", "inherited", "dirty".

The "inherited" means that the branch cache is not only "clean", but it is a
duplicate of its parent filter.

If a branch cache is "inherited", we can non only skip writing its value on
disk, but it is a good idea to delete any stale value on disk, as those will
just waste time (and possibly induce bug) in the future.

We only do this in the update related to transaction or explicit cache update
(e.g `hg debugupdatecache`). Deleting the file when we simply detected a stall
cache during a read only operation seems more dangerous.

We rename `copy` to `inherit_for` to clarify we associate a stronger semantic
to the operation.