Different RestxSession cookie names for different apps

When developping several restx application (thus, working on `localhost` domain), restx cookies are interfering since their names are the same.

Proposition here, is to define an `appName` in `SignatureKey`, and suffix RestxSession & RestxSessionSignature cookies with this appName, thus allowing to work on 2 different restx app on localhost, at the same time (different cookies).

Backward compatibility is kept for the moment (not providing any appName in SignatureKey is allowed, and this is behaving the same as not providing any SignatureKey at all) : in that case, non-suffixed cookie names (old `RestxSession` and `RestxSessionSignature`) are generated.
But I'm wondering if we shouldn't discourage not adding any appName (ATM, SignatureKey(byte[]) is only deprecated, but I'm wondering if we shouldn't remove it purely and simply and use a `noname` appName in that case)