Improve koa example

[danneu]

I was passing through and noticed that the koa example was severely out of date, so I cleaned it up and made it much more koa-idiomatic.

Mainly,

• Removed redundant try/catch error handlers since koa already lifts uncaught errors into 500 responses.
• Flattened the boot procedure
• Ensured that the RethinkDB connection always closes even if downstream handlers fail.

[danielmewes]

Thanks for the PR @danneu !

@segphault or @deontologician is one of you available for reviewing this change?

[deontologician]

I will leave it to segphault, I haven't ever used koa myself