Cap maximum number of CDC requests

No point letting huge queues build -- especially now that we
deduplicate client requests.

cpp/cdc/CDC.hpp

@@ -15,6 +15,8 @@ struct CDCOptions {
     bool syslog = false;
     Duration shardTimeout = 100_ms;
     bool xmon = false;
+    // at 1ms per request it's 10 seconds worth of requests.
+    uint64_t maximumEnqueuedRequests = 10000;
 };
 
 void runCDC(const std::string& dbDir, const CDCOptions& options);