Fix connection management for noTimeoutClient

1. Don't share a connection pool between DefaultDockerClient.client and
   noTimeoutClient. Each Jersey client should have its own connection pool.

2. Shut down noTimeoutClient as part of DefaultDockerClient.close, just
   like we do for the default client.

3. Get rid of an ExecutorService that is no longer being used.

4. Fix races in DefaultDockerClientTest.testConnectionRequestTimeout that
   caused it to sometimes exit without doing anything useful or throwing
   the expected exception.
Remove unused imports
Merge pull request #82 from spotify/rohan/cleanup

Fix connection management for noTimeoutClient