[qa] Change sync_blocks to pick smarter maxheight

Instead of syncing to max height returned by the waitforblockheight RPC, sync
to the max height returned by the getblockcount RPC.

This change was suggested by Suhas Daftuar <sdaftuar@chaincode.com>.

qa/rpc-tests/test_framework/util.py

@@ -129,17 +129,19 @@ def sync_blocks(rpc_connections, *, wait=1, timeout=60):
     one node already synced to the latest, stable tip, otherwise there's a
     chance it might return before all nodes are stably synced.
     """
-    maxheight = 0
+    # Use getblockcount() instead of waitforblockheight() to determine the
+    # initial max height because the two RPCs look at different internal global
+    # variables (chainActive vs latestBlock) and the former gets updated
+    # earlier.
+    maxheight = max(x.getblockcount() for x in rpc_connections)
     start_time = cur_time = time.time()
     while cur_time <= start_time + timeout:
         tips = [r.waitforblockheight(maxheight, int(wait * 1000)) for r in rpc_connections]
-        heights = [t["height"] for t in tips]
-        if all(t == tips[0] for t in tips):
-            return
-        if all(h == heights[0] for h in heights):
+        if all(t["height"] == maxheight for t in tips):
+            if all(t["hash"] == tips[0]["hash"] for t in tips):
+                return
             raise AssertionError("Block sync failed, mismatched block hashes:{}".format(
                                  "".join("\n  {!r}".format(tip) for tip in tips)))
-        maxheight = max(heights)
         cur_time = time.time()
     raise AssertionError("Block sync to height {} timed out:{}".format(
                          maxheight, "".join("\n  {!r}".format(tip) for tip in tips)))