Improved Memory Management for ClickHouse Keeper Watch Callbacks

This optimizes the interaction with ClickHouse Keeper. Previously the caller could register the same watch callback multiple times. In that case each entry was consuming memory and the same callback was called multiple times which didn't make much sense. In order to avoid this the caller could have some logic to not add the same watch multiple times. With this change this deduplication is done internally if the watch callback is passed via shared_ptr. #53452 (Alexander Gololobov).