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 don't think that's necessary. The connection managers don't preallocate any connections, so when they're created, they're just empty pools.

Additionally, if there's an exception and `DefaultDockerClient` isn't constructed, there will be no reamining references to `cm` or `noTimeoutCm` and they will be garbage-collected. That will invoke `PoolingHttpClientConnectionManager.finalize` -> `shutdown` anyway.