Fix Flux.merge() concurrency limit in SubscribeChainStatus (#792)

src/main/kotlin/io/emeraldpay/dshackle/rpc/SubscribeChainStatus.kt

@@ -25,10 +25,9 @@ class SubscribeChainStatus(
     }
 
     fun chainStatuses(): Flux<BlockchainOuterClass.SubscribeChainStatusResponse> {
-        return Flux.merge(
         // we need to track not only multistreams with upstreams but all of them
         // because upstreams can be added in runtime with hot config reload
-            multistreamHolder.all()
+        val sources = multistreamHolder.all()
             .filter { Common.ChainRef.forNumber(it.getChain().id) != null }
             .map { ms ->
                 Flux.concat(
@@ -40,8 +39,9 @@ class SubscribeChainStatus(
                         multistreamEvents(ms),
                     ),
                 )
-                },
-        ).doOnError {
+            }
+        return Flux.merge(Flux.fromIterable(sources), sources.size)
+            .doOnError {
                 log.error("Error during sending chain statuses", it)
             }
     }