quiche-server ignores POST request payloads

The quiche-server currently ignores POST request payloads and decides the response based on headers alone (https://github.com/cloudflare/quiche/blob/master/apps/src/common.rs#L1555). It would be helpful for the server to properly consume and drain the stream for tests involving POST requests (#1400).

I started to implement this a while ago so feel free to reference but I don't think I can carry this PR through right now: https://github.com/cloudflare/quiche/compare/master...ygina:quiche:http-post?expand=1