Redo SequenceServer.run.

We depend on Thin now.

As per standing discussion on issue #89, using Thin should workaround that
issue.

We now tell users SequenceServer is ready when it _really is_. Previously we
would optimistically announce SequenceServer is ready and then crash on
finding that another process is using SequenceServer's bind URI.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>