[fix][broker][branch-2.8] Fix watcher count on ZK increasing indefinitely

[coderzc]

Motivation

I find some z-node only call exist and add watcher, but ExistsWatchRegistration still will be registered even though the z-node may never be created. this will lead to the watcher count on ZK increasing indefinitely.

    /** Handle the special case of exists watches - they add a watcher
     * even in the case where NONODE result code is returned.
     */
    class ExistsWatchRegistration extends WatchRegistration {

        public ExistsWatchRegistration(Watcher watcher, String clientPath) {
            super(watcher, clientPath);
        }

        @Override
        protected Map<String, Set<Watcher>> getWatches(int rc) {
            return rc == 0 ? watchManager.dataWatches : watchManager.existWatches;
        }

        @Override
        protected boolean shouldAddWatch(int rc) {
            return rc == 0 || rc == KeeperException.Code.NONODE.intValue();
        }

    }

Modifications

Remove watcher if exist return NONODE.
Remove unused method in ZkUtils.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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