Clean up Public Client Bootstrap API

[Enrico Olivelli]

Hi BookKeepers,
While reviewing PR for "BOOKKEEPER-612: Region aware placement" I have 
noticed the introduction of an hard dependency to netty3 in the Public API.

I have just created this issue 
https://issues.apache.org/jira/browse/BOOKKEEPER-942 "Clean up Public 
Client Bootstrap API" to clean up the API in order to have a more 
future-proof and sexy API

Maybe we can work on it for 4.6.0 as for 4.5.0 there is still a lot of 
work to be done

-- Enrico