short delay before retrying blockheight RPC call

jmclient/jmclient/wallet_service.py

@@ -808,10 +808,15 @@ def sync_unspent(self):
         # block height needs to be real time for addition to our utxos.
         # We are a little aggressive in trying this RPC call, because if
         # it fails we cannot continue:
-        for _ in range(5):
+        for i in range(5):
             current_blockheight = self.bci.get_current_block_height()
             if current_blockheight:
                 break
+            if i < 4:
+                jlog.debug("Error calling blockheight RPC, trying again in 0.5s.")
+            # sync unspent calls are synchronous; this short sleep should
+            # not affect us since we are not yet in main monitor loop.
+            time.sleep(0.5)
         if not current_blockheight:
             # this failure is critical; give up:
             jlog.error("Failed to access the current blockheight, "