Fixes https://github.com/denoland/deno/issues/18700
Timeline of the events that lead to the bug.
1. WebSocket handshake complete
2. Server on `read_frame` holding an AsyncRefCell borrow of the
WebSocket stream.
3. Client sends a TXT frame after a some time
4. Server recieves the frame and goes back to `read_frame`.
5. After some time, Server starts a `write_frame` but `read_frame` is
still holding a borrow!
^--- Locked. read_frame needs to complete so we can resume the write.
This commit changes all writes to directly borrow the
`fastwebsocket::WebSocket` resource under the assumption that it won't
affect ongoing reads.
