feat(http): support mounting on subroutes

[benjie]

• Gets rid of /_postgraphile/* (backwards compatible for /_postgraphile/stream in case you hard-coded that (don't!))
• Stream URL now at /graphql/stream; but don't hard-code that - we set it in the X-GraphQL-Event-Stream header which you should use instead
• Switches to using originalUrl rather than url when processing the request
• graphqlRoute and graphiqlRoute are treated as absolute paths - i.e. app.use('/path/to', postgraphile()) will not work, you need to do: app.use('/path/to', postgraphile({graphqlRoute: '/path/to/graphql'}))
• Even works with Koa/koa-mount
• Fixes incorrect type for enableQueryBatching