Create first cache generation by default when the cache is started

[cabol]

Currently, when the local cache is started, the first generation is not created by default, hence if we perform some operation in the cache, an exception is raised. To avoid this exception we have to call MyCache.new_generation() before to use the cache or set the option gc_interval in the config.

The idea is to avoid this situation by creating the first cache generation automatically when the cache is started; maybe in the Nebulex.Adapters.Local.Generation.init callback. Hence, if we don't set the option gc_interval in the config because maybe we want to create generations in a different way using other strategy, etc., at least the first generation will be created and we won't get any exception.