Fixed bug in io::poll.

When a future returned from process::io::poll is discarded we need to
fully remove the ev_io watchers from libev otherwise the reuse of
those file descriptors can cause untended side effects because they're
still being used by the kernel/libev.

Review: https://reviews.apache.org/r/24194
Redirect docker logs

Review: https://reviews.apache.org/r/24464
Add new Docker configurations

Review: https://reviews.apache.org/r/24475
Added Docker::pull