v1.8.20: Optimize Native Compression Performance & Fix Configuration Handling

Changed

• The compression middleware on the native engine no longer wraps res.json and res.send per request: it hands its settings to the response, and the reply takes the server's own buffered-response compression, the path a performance.compression setting takes. One property write per request, the response keeps its shape, and a route-level compression() costs what a server-level setting does. A filter is consulted per response, so a middleware with one keeps the wrapping path, as the middleware does on the other servers. Measured on the HttpArena json-comp mix at one worker, the route goes from 7% behind the fastest uWebSockets.js entry to 8% ahead of it.
• Which kept encoder a settings object uses is settled once per level instead of being keyed by a serialised options object on every reply, and Accept-Encoding is parsed once per distinct header string per preference list (a bounded memo). isCompressible answers application/json and text/html before parsing the type.

Fixed

• performance.compression.gzip, .brotli and .reuseEncoders reach the servers: the config validator dropped them, so a brotli quality pinned there compressed at the quality the shared level maps to (level 6 is quality 7), and the kept-encoder switch had no effect.