perf(websocket): use bound callbacks

Instead of allocating one temporary function for each WebSocket
`send()` call.

Regarding the test removal, the permessage-deflate threshold was
implemented in the "ws" package in [1], so it's not needed anymore.

[1]: https://github.com/websockets/ws/commit/6b3904b42dbd48aed2e0d8d599787cca04f05384