HTTPCLIENT-1454: (re)Introducing HttpClientConnectionOperator in public API

In some edge cases, possibility to provide own implementation of
HttpClientConnectionOperator is desired, but in general, HttpClient
users should not mangle it, as the default provided by client
is completely satisfying.

Related issue
https://issues.apache.org/jira/browse/HTTPCLIENT-1454

Makes PR obsolete (supersedes it)
https://github.com/apache/httpclient/pull/9