Fixed JERSEY-2222 & JERSEY-2189.

- JERSEY-2222: Support ClientProperties.ASYNC_THREADPOOL_SIZE.
- JERSEY-2189: Allow to remove User-Agent header entirely in Client requests.

Change-Id: Ifef9f43551d09d5ee02426a6ab7269adc6ba4645
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>
Merge "Fixed JERSEY-2222 & JERSEY-2189."