Use fastutil's LongSet in `Sink.dedupSet`

FYI @kaijianding

Also, consider using a proven (SMHasher-wise) hashing algorithm such as xxHash (implemented e. g. in https://github.com/OpenHFT/Zero-Allocation-Hashing) rather than BKDRHash, which seems like a marginal improvement (if improvement at all) over the String's default hash code (the only difference is using 131 instead of 31 as a multiplication constant). While String's default hash function is widely considered a bad hashing algorithm.

Related: #6861

Start by locating the Sink.dedupSet entry point and inspect how it currently stores and hashes values. Read related issue #6861, then determine whether the requested change is limited to using fastutil's LongSet or also includes the xxHash/BKDRHash concern. Done means the agreed scope is implemented and deduplication behavior remains covered.

Written by the indexing model from the issue text.