ISSUE-577: Clean up internal Bookkeeper constructors

[sijie]

Original Issue: apache#577

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JIRA: https://issues.apache.org/jira/browse/BOOKKEEPER-1011

Reporter: Robert Joseph Evans @revans2

One of the feedbacks from

apache#105 (comment)

was that there are some customer APIs that need to be cleaned up. This is specifically for the two constructors in Bookkeeper

{code}
public BookKeeper(ClientConfiguration conf, ZooKeeper zk);
public BookKeeper(ClientConfiguration conf, ZooKeeper zk, ClientSocketChannelFactory channelFactory);
{code}

That are really for internal use only. We should find a way to mark them as such.