This PR adds a newframework.router.cache_dir configuration setting. It will default tokernel.cache_dir as previously, but makes it possible to easily re-configure the Router's cache directory.

My intended use case is to set it to~ (null) for thetest environment because I need to modify or otherwise dynamically load routes during tests. Another use case I see for it is to have tenant-specific route configurations and caches in multi-tenancy applications.