Fixed JERSEY-2424.

- Added static methods for retrieving underlying HTTP client instances to all existing ConnectorProvider implementations
- Added unit tests covering the issue.
- Introduced new Initializable API in Jersey client module, which is used by the fix implementation.

Change-Id: Ib1e806103221a7a35c54347ed665277178706333
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>
Merge "Fixed JERSEY-2424."