[exp] add limiter2 to better protect against bots
- limiter2 from https://gist.github.com/dalf/76ee391b6353f979981ae40af9b79132 (impl. by dalf)
- can be enabled by setting LIMITER2 to true and adding REDIS to the stack
- only limiter or limiter2 can be enabled
- this feature is still exp. and impl. as a test; evetually limiter2 as well as filtron are going to be droped and replaced by a proper limiter impl.
- setting LIMITER2 to true will also enable the default plugins in settings.yml