Netty Headers Class Restructure and Algorithm Updates

Motivation:
Headers within netty do not cleanly share a common class hierarchy.  As a result some header types support some operations
and don't support others.  The consolidation of the class hierarchy will allow for maintenance and scalability for new codecs.
The existing hierarchy also has a few short comings such as it is not clear when data conversions are happening.  This
could result unintentionally getting back a collection or iterator where a conversion on each entry must happen.

The current headers algorithm also prepends all elements which means to find the first element or return a collection
in insertion order often requires a complete traversal followed by a collections.reverse call.

Modifications:
-Provide a generic base class which provides all the implementation for headers in netty
-Provide an extension to this class which allows for name type conversions to happen (to accommodate legacy CharSequence to String conversions)
-Update the headers interface to clarify when conversions will happen.
-Update the headers data structure so that appends are done to avoid unnecessary iteration or collection reversal.

Result:
-More unified class hierarchy for headers in netty
-Improved headers data structure and algorithms
-headers API more clearly identify when conversions are required.
@trustin I agree that it may simplify the interface in cases where we want to use `Object` and generic type `T` in the same type of signature.  However its sounds like the implementation may get a bit messy by constantly doing converstions, having casts (and exception cases) everywhere, or both.  Also using the generics allows us to not *have* to do translations or extra method calls in the primary use case (getting back the type which is stored in the map).  The current interface (although admittedly a bit clunky to have to have `object` on the method name) also clarifies that a translation will be happening.  There are a bunch of cases in this PR where there were translations done that didn't require being done...and this was just in Netty's own code.  I think I would rather stick with the current approach for these reasons...does that work?