Replace interface+impls with a single concrete container
which wraps a map.
This breaks code which was sub-classing.
However, no external code can be found which does this.
A build() method is used instead of simple ctor so
that the ctor is private, which will cause theoretical
external sub-class code to fail to compile.
Code using getChannelProviderRegistry()
and existing methods will behave as before.
Add ChannelProviderRegistry::getFactory() for pass through
to ChannelProviderFactory (Registery is just a proxy()...)
Remove existing get/create virtual method in favor of getFactory.
Update both existing implementations.
Add SimpleChannelProviderFactory to cut down on boilerplace
for the common case.
Add method to clear mapping and release any saved sharedInstance.
