Default rejected-handler will leak buffers in raw stream HTTP servers

# Problem

Given an HTTP server started with `:raw-stream? true`, when `aleph.http.server/handle-request` hits the `RejectedExecutionException` path, the default handler (i.e. when no custom `rejected-handler` was provided) will leak any byte buffers which are already present in the request's `:body` stream.

# Possible solutions

1. Drain the body channel and release all byte buffers still contained in it (does that cover all bases?)
2. Document this fact (as suggested by @arnaudgeiser [here](https://github.com/clj-commons/aleph/issues/615#issuecomment-1207209343))
3. Require passing a custom `rejected-handler` when `:raw-stream? true` is passed

Start at aleph.http.server/handle-request and trace the RejectedExecutionException path for servers using :raw-stream? true. Reproduce the default rejected-handler case, inspect ownership of byte buffers already in the :body stream, and establish which proposed behavior prevents leaks; done means the rejected request releases retained buffers without requiring an undocumented custom handler.

Written by the indexing model from the issue text.