Improve JUnit support

These are small improvements to merged JUnit support for Merge Requests:

• We should not send Poll-Interval when data is returned. It makes FE to periodically re-request it. Polling should happen while we are parsing,
• We might want to figure out how we support pipeline when we retry jobs. It seems that test results will be cached for them, but not refreshed,
• We might want to reduce caching time for 10 minutes, as I'm not sure what impact it is gonna have on Redis,
• Fix runner features support for shells: gitlab-runner!989 (merged),
• Documentation: https://gitlab.com/gitlab-org/gitlab-ce/issues/49867,
• Feature specs FE+BE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20972,
• Development fixtures: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20973