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.
I would argue no since once we pass the connection managers to the Jersey, the Jersey clients are responsible for their state. Calling `client.close()` on a Jersey client closes the underlying connection manager, and we do that in `DefaultDockerClient.close`.