fix(koa): don't hang when body is already parsed

[benjie]

Previously if you had bodyParser in your Koa middleware stack before PostGraphile, e.g.

const Koa = require('koa');
const bodyParser = require('koa-bodyparser');
const { postgraphile } = require('postgraphile');

const app = new Koa();
app.use(bodyParser()); // < this!
app.use(postgraphile());
app.listen(3000);

then PostGraphile (well, strictly it was body-parser itself) would hang whilst processing GraphQL requests because it didn't realise that the request body had already been consumed.

I've added some additional middleware which converts the Koa body parsing over to something that looks like Connect body parsing (it has to manipulate the ctx.req object to achieve this - sorry!); this should solve this issue.

Remember: Koa support is experimental! When raising issues please include a minimal reproduction test case (such as the one above) to help me get to the root of the issue faster.

Fixes #602