Add a "relaunch" API for a session

Currently we have the "restart" API for _running_ sessions.Let's add a new API to "relaunch" a session which has been terminated.It should keep the session ID same.

This will be a follow-up to #325 because the session record should be kept intact while the kernel records should be recreated.

TODO: Should we preserve the old kernel records? They may be still required to calculate billing and usage statistics, even though the task results are failures.

JIRA Issue: BA-278