Snapshot sync and block gap info in eth_syncing (#2948)

* provide snapshot sync info in eth_syncing

* specify block gap in eth_syncing

* Extend eth_syncing with warp, format the output properly

* adjust serialization tests for sync info

* whitespace

js/src/api/format/output.js

@@ -153,6 +153,28 @@ export function outSignerRequest (request) {
   return request;
 }
 
+export function outSyncing (syncing) {
+  if (syncing && syncing !== 'false') {
+    Object.keys(syncing).forEach((key) => {
+      switch (key) {
+        case 'currentBlock':
+        case 'highestBlock':
+        case 'startingBlock':
+        case 'warpChunksAmount':
+        case 'warpChunksProcessed':
+          syncing[key] = outNumber(syncing[key]);
+          break;
+
+        case 'blockGap':
+          syncing[key] = syncing[key].map(outNumber);
+          break;
+      }
+    });
+  }
+
+  return syncing;
+}
+
 export function outTransaction (tx) {
   if (tx) {
     Object.keys(tx).forEach((key) => {