feat(watch): add Watch.Watcher#cancel to cancel a single watcher

[chaofengliu-okg]

scripts: add Watch.Watcher#cancel to cancel a single watcher

Motivation
Currently, closing a watcher is only possible via Watcher#close, which merely sends an HTTP/2 END_STREAM frame. However, etcd does not terminate the underlying goroutines and watcher stream in response, which can lead to memory leaks.

Solution
By adding a cancel function that directly sends an HTTP/2 RST_STREAM frame, etcd can properly shut down the goroutines and the watcher stream.

Reproduction

for () {
    Watch.Watcher watcher = watchClient.watch(
        key,
        (WatchResponse resp) -> {
            System.out.println("Received event: " + resp);
        },
        (Throwable t) -> {
            System.out.println("Error: " + t);
        },
        () -> {
            System.out.println("Completed");
        }
    );
    // watcher.cancel();
    watcher.close();
    System.in.read();
}

After Change
After adding the cancel API, replacing watcher.close() with watcher.cancel() allows the etcd server to properly release the stream and goroutines.

Fixes #1510
@fanminshi @lburgazzoli @vorburger

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[lburgazzoli]

should this be part of close ?

[chaofengliu-okg]

If we alter the close implementation, could this break existing user dependencies on the current close behavior?

[lburgazzoli]

I think this is a leftover from the original design where we were supposed to re-use the same stream across multiple watcher, but this is not the case as today, so I'd for merging the functions.

[chaofengliu-okg]

I've changed the modification status from 'cancel' to 'close'. Please review again.

[lburgazzoli]

@chaofengliu-okg is there any way to add a test for this case ?

[chaofengliu-okg]

@lburgazzoli I have added the unit tests.

[chaofengliu-okg]

​​Thank you for merging the PR. I would like to ask about the schedule for the next release.​
@lburgazzoli