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This is a forward port (from scylla-enterprise) of additional compression options (zstd, dictionaries shared across messages) for inter-node network traffic. It works as follows: After the patch, messaging_service (Scylla's interface for all inter-node communication) compresses its network traffic with compressors managed by the new advanced_rpc_compression::tracker. Those compressors compress with lz4, but can also be configured to use zstd as long as a CPU usage limit isn't crossed. A precomputed compression dictionary can be fed to the tracker. Each connection handled by the tracker will then start a negotiation with the other end to switch to this dictionary, and when it succeeds, the connection will start being compressed using that dictionary. All traffic going through the tracker is passed as a single merged "stream" through dict_sampler. dictionary_service has access to the dict_sampler. On chosen nodes (in the "usual" configuration: the Raft leader), it uses the sampler to maintain a random multi-megabyte sample of the sampler's stream. Every several minutes, it copies the sample, trains a compression dictionary on it (by calling zstd's training library via the alien_worker thread) and publishes the new dictionary to system.dicts via Raft's write_mutation command. This update triggers (eventually) a callback on all nodes, which feeds the new dictionary to advanced_rpc_compression::tracker, and this switches (eventually) all inter-node connections to this dictionary. Closes scylladb/scylladb#22032 * github.com:scylladb/scylladb: messaging_service: use advanced_rpc_compression::tracker for compression message/dictionary_service: introduce dictionary_service service: make Raft group 0 aware of system.dicts db/system_keyspace: add system.dicts utils: add advanced_rpc_compressor utils: add dict_trainer utils: introduce reservoir_sampling utils: introduce alien_worker utils: add stream_compressor
