fix: websocket session and callback promise retention leak

[VastBlast]

This PR fixes #153, a client-side memory leak in long-lived RPC sessions.

The session read loop was racing each transport.receive() call against a single session-long abort promise, which caused Promise.race() to accumulate until the entire session was disposed. This changes RpcSession to use a fresh cancellation promise for each pending read.

Edit: I also noticed another issue with callbacks causing a memory leak which isn't specifically reproduced in #153 which has been patched in this PR too.

Essentially if you had something like this where the callback is called regularly:

subscribeToTicks(cb: RpcStub<(ts: number) => void>) {
    const callback = cb.dup();

    let running = true;
    (async () => {
        while (running) {
            callback(Date.now());
            await new Promise(resolve => setTimeout(resolve, 100));
        }
    })()

    function cleanup() {
        running = false;
        callback[Symbol.dispose]?.();
    }

    callback.onRpcBroken(() => { cleanup(); });
}

The client would subscribe and capnweb would keep the callback promises in memory even though the result cannot be used.

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Latest commit: 4fc13b0

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