Vapor: 4.13.1 Release

This patch was authored and released by @tanner0101.

Refactors HTTP request body decoding to immediately make streaming bodies available via req.body.drain (#2413).

Previously, Vapor's HTTP request decoder would wait for a second body chunk to arrive before initiating body streaming. This was done as a performance optimization for single-chunk bodies since streaming has overhead. However, this behavior made it difficult to implement real time streaming handlers, like a ping/pong handler.

The HTTP request decoder has been updated to initiate body streaming immediately upon receiving the first chunk. To avoid impacting performance on small, non-streaming request bodies, a check for content-length has been added. If the request's body is contained entirely in a single chunk, streaming overhead is avoided.

Below are performance numbers on Ubuntu 20.04 in req/s.

|Method|Body Strategy|Previous|New|Delta| |-|-|-|-|-| |GET|.collect|229979.85|229333.16|0.28%| |POST|.collect|198949.90|196567.85|1.21%| |POST|.stream|197916.54|196178.48|0.89%|

The numbers show a negligible change in performance.