[server][client] Add cooperative KV backpressure based on RocksDB L0

[platinumhamburg]

Introduce a two-tier backpressure model to keep RocksDB write stall from blocking RPC handler threads:

• Tier 1 (piggyback): server reports a normalized [0, 1) pressure on PbPutKvRespForBucket once L0 reaches kv.backpressure.l0-slowdown-trigger (default 12); client throttles per bucket with max_throttle_ms * p^3 via client.writer.kv-backpressure.max-throttle-ms (default 5000ms).
• Tier 2 (hard rejection): once L0 reaches RocksDB's level0_slowdown_writes_trigger, server throws StorageBackpressureException (Errors code 71, RetriableException); client applies the same max-throttle window as backoff before retry.

Add maintenance configuration doc for the new server option.

Purpose

Linked issue: close #3454 3454

Brief change log

Tests

API and Format

Documentation