Finer priority management for interceptors

[gdrouet]

Currently we can configure Atmosphere to execute two interceptors before/after the defaults but we can't change the order between them.

[gdrouet]

@jfarcand I just draft a possible refactoring for invocation order. The idea is to make possible to specify the order of invocation between two interceptors that are configured to be executed after default or before default interceptors.

This draft also gives the possibility to simplify AtmosphereFramework class:

• manage only one list of interceptors well ordered (default + non default)
• hide the ordering in the Priority class

This approach also introduces new rules:

• An InvocationOrder IS-A AtmosphereInterceptor
• Any default AtmosphereInterceptor IS-A InvocationOrder

Take a look at the code and tell me if I'm going in the wrong way. Will continue if you're agree with this.

[gdrouet]

Well, by using meta services provided in #1573, I can order additional interceptors as required. I won't work on it while it is YAGNI. Even if it could be a good internal enhancement, it doesn't bring additional value for the user for now. There is a big challenge to not break anything, we should work on it only if we really need it!