Fixed JERSEY-2205, JERSEY-2258 & JERSEY-2261.

- JERSEY-2261: Executor provider SPI & impl refactoring.
- JERSEY-2205: properly closing async executors.
- JERSEY-2258: (relates to JERSEY-2222), Introduced ConnectorProvider SPI and refactored Connector implementations.

- Other changes:
  - Updated Jersey test framework to properly dispose test client instances.
  - Bumped Grizzly server version to 2.3.8 
    - Removed stale Grizzly module references from our OSGi test configurations
    - Replaced deprecated HttpServer.stop() calls
  - Bumped Grizzly async client version to 1.6

Change-Id: I4f134c50dfb11e130842a25cd47531eec48ca7b8
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>