Add global concurrency limiter for CDC queries across all workers

Problem

When a CDC consumer starts up (especially for the first time), all workers begin querying the cluster simultaneously with no global concurrency control. The current execution model uses CompletableFuture chaining on a ScheduledExecutorService, meaning a single thread can have many outstanding async requests. The executor pool size (default: availableProcessors - 1) limits thread count but not in-flight queries.

On a cluster with 256 vnodes, this can result in 250+ concurrent CDC log readers overwhelming the cluster with queries.

Existing mechanisms (insufficient)

• WorkerConfiguration.withMinimalWaitForWindowMs() — adds a static delay per worker but doesn't limit aggregate concurrency
• Connection pool limits (poolingMaxRequestsPerConnectionLocal, poolingMaxQueueSize) — act as backstops but allow far too many concurrent requests in aggregate
• ExponentialRetryBackoffWithJitter — only triggers on errors, by which point the cluster is already stressed

Proposal

Add a shared concurrency limiter (e.g., a Semaphore) across all workers to cap the number of concurrent in-flight CDC queries.

Suggested approach

• Add a configurable max concurrent queries option to WorkerConfiguration.Builder (e.g., withMaxConcurrentQueries(int max))
• Implement via a shared Semaphore that workers acquire before issuing a CDC query in TaskAction and release on completion
• A sensible default (e.g., 32–64) would prevent cluster overload while still allowing reasonable throughput
• The semaphore should be shared across all workers within a single CDCConsumer instance

Key files

• scylla-cdc-base/src/main/java/com/scylladb/cdc/model/worker/WorkerConfiguration.java — add config option
• scylla-cdc-base/src/main/java/com/scylladb/cdc/model/worker/TaskAction.java — add semaphore gating around query execution
• scylla-cdc-lib/src/main/java/com/scylladb/cdc/lib/CDCConsumer.java — expose in the public builder API

Related

• scylladb/scylla-cdc-source-connector#235 — connector-side issue for exposing this as a connector config property

Start by reading WorkerConfiguration.java and TaskAction.java to trace how workers issue and complete CDC queries, then inspect CDCConsumer.java for the public builder flow. Define how one limiter is shared across a CDCConsumer instance, how configuration and defaults are exposed, and verify that permits cover every in-flight query and are released on completion.