Add preciousblock tests

Rebased, improved and extended by Luke-Jr.

qa/rpc-tests/test_framework/util.py

@@ -133,6 +133,16 @@ def sync_blocks(rpc_connections, wait=1, timeout=60):
         timeout -= wait
     raise AssertionError("Block sync failed")
 
+def sync_chain(rpc_connections, wait=1):
+    """
+    Wait until everybody has the same best block
+    """
+    while True:
+        counts = [ x.getbestblockhash() for x in rpc_connections ]
+        if counts == [ counts[0] ]*len(counts):
+            break
+        time.sleep(wait)
+
 def sync_mempools(rpc_connections, wait=1, timeout=60):
     """
     Wait until everybody has the same transactions in their memory