Resolve the issue of not being able to produce a block

Author: chopin-fan

src/AElf.Kernel.Consensus.AEDPoS/Application/IrreversibleBlockHeightUnacceptableLogEventProcessor.cs

@@ -51,14 +51,17 @@ protected override async Task ProcessLogEventAsync(Block block, LogEvent logEven
         var distanceToLib = new IrreversibleBlockHeightUnacceptable();
         distanceToLib.MergeFrom(logEvent);
 
-        if (distanceToLib.DistanceToIrreversibleBlockHeight > 0)
         {
+            var chain = await _blockchainService.GetChainAsync();
             Logger.LogDebug($"Distance to lib height: {distanceToLib.DistanceToIrreversibleBlockHeight}");
+            if ( block.Height - chain.LastIrreversibleBlockHeight < distanceToLib.DistanceToIrreversibleBlockHeight/2)
+            {
+                return;
+            }
             Logger.LogDebug("Will rollback to lib height.");
             _taskQueueManager.Enqueue(
                 async () =>
                 {
-                    var chain = await _blockchainService.GetChainAsync();
                     await _blockchainService.ResetChainToLibAsync(chain);
                 }, KernelConstants.UpdateChainQueueName);
         }