Allow for safe coarser-scope shared AsyncHttpClient creation for which no client and its options builder exists. 

Hi Jeanfrancois

I think we should provide (i.e. I need :) ) a way to create a "globally" shared ```AsyncHttpClient``` for which no socket(s) or clients exist yet, but which is subject if being passed to (and thus shared by) all subsequent Socket creations as the shared, underlying runtime. 
 
In addition there should be the possibility of configuring the runtimes' ```NioClientSocketChannelFactory``` as 
well. 

Cheers, Christian.