problem: doesn't get head updates from all available heads

solution: ensure all heads are started on multihead start
rel: #67

src/main/kotlin/io/emeraldpay/dshackle/upstream/MergedHead.kt

@@ -33,11 +33,18 @@ class MergedHead(
     }
 
     override fun start() {
+        sources.forEach { head ->
+            if (head is Lifecycle && !head.isRunning) {
+                head.start()
+            }
+        }
+        subscription?.dispose()
         subscription = super.follow(Flux.merge(sources.map { it.getFlux() }))
     }
 
     override fun stop() {
         subscription?.dispose()
+        subscription = null
     }
 
     override fun setCaches(caches: Caches) {

src/main/kotlin/io/emeraldpay/dshackle/upstream/ethereum/EthereumMultistream.kt

@@ -91,7 +91,8 @@ open class EthereumMultistream(
             upstream.setLag(0)
             upstream.getHead()
         } else {
-            val newHead = MergedHead(upstreams.map { it.getHead() }).apply {
+            val heads = upstreams.map { it.getHead() }
+            val newHead = MergedHead(heads).apply {
                 this.start()
             }
             val lagObserver = EthereumHeadLagObserver(newHead, upstreams as Collection<Upstream>).apply {

src/main/kotlin/io/emeraldpay/dshackle/upstream/ethereum/EthereumWsHead.kt

@@ -33,6 +33,7 @@ class EthereumWsHead(
     }
 
     override fun start() {
+        this.subscription?.dispose()
         this.subscription = super.follow(ws.getFlux())
     }