Timeouts
    * slow responses? (feasable? just let TCP do it?)

streamed responses

    * instead of an implicit "low water mark" of 0 for the poll_cb
    writer-object callback, a configurable number of bytes can be used.

Handle requests that don't require a body (optional entities).

    * Related: allow overriding the "if entity has a C-L, wait for it" during request
    start (not sure if there's a PSGI-compatible way to do this)
    * different request timeout logic will be needed.

Optimize env-hash

    * allow Feersum apps to specify which vars will be needed (v1.1)
    ** will this even be a win?
    * maybe use uvar magic for PSGI if using Perl v5.10.0 (Variable::Magic
    style?) (v1.1)

Perf idea: Un-corked reads - should newly accepted handles wait or just try
read right away?

multiple Feersum threads, one Perl thread?

WebSocket handshake acceleration in XS
    * Currently supported via H2 Extended CONNECT tunnels (RFC 8441) and psgix.io
    * Accelerating the H1 handshake in C/XS would reduce overhead

JSON accelleration?
    * can write_json() an element in an array of things? automagically call JSON::XS stuff?
    * middleware with XS-accellerator?
