Close event connections when the server is gracefully shutting down.

@airhorns for review

## Problem

When the TERM signal is sent to TERM, then it waits for existing connections to finish before exiting (https://github.com/macournoyer/thin/blob/v1.6.2/lib/thin/server.rb#L175).  There is a 30 second timeout for most requests, but the [/events endpoint](https://github.com/Shopify/dashing/blob/v1.3.4/lib/dashing/app.rb#L67-75) uses `stream :keep_open do |out|` to keep the connection open for server sent events.  As a result, a graceful shutdown can take over an hour to complete.

## Solution

I couldn't find a simple hook in thin to close connections when the server is shutting down, but I ended up wrapping the Thin::Server#stop method to close the event connections.

I tested manually by sending the TERM signal to the thin processes in development after visiting a page which connected to the /events endpoint.  Previously this would get stuck waiting until I close the page, but now it completes quickly.
This didn't work when there was more than one connection, because the [callback on line 74](https://github.com/Shopify/dashing/pull/383/files#diff-ef95f95dc35a6b73d99a51e32d90ec48R74) deletes the connection from the array, which results in `.each` here skipping connections.  I pushed commit 55f90939eae4d6eb64822fd3590f694418396510 to dup the array before iterating on it here.