Skip Interlocked.Exchange and FlushConfig when no update detected

[vazois]

This PR fixes an issue that caused message flooding across the cluster. This happened because the config object kept being updated even when the received config did not change the local config during merge.

The check for an updated config is happening implicitly by comparing the new and current config objects

garnet/libs/cluster/Server/Gossip.cs

Lines 116 to 123 in af9cf0e

	var currentCopy = current.Copy();
	var next = currentCopy.Merge(senderConfig, workerBanList, logger).HandleConfigEpochCollision(senderConfig, logger);
	if (currentCopy == next) return false;
	if (Interlocked.CompareExchange(ref currentConfig, next, current) == current)
	break;
	}
	FlushConfig();
	return true;

This meant that the call to currentCopy.Merge should return the same object if no update has happened. This occurs correctly when merging the worker info

garnet/libs/cluster/Server/ClusterConfig.cs

Line 917 in af9cf0e

if (worker.ConfigEpoch <= workers[i].ConfigEpoch) return this;

However, an equivalent check was not there when the object was updated during the merging of the slotMap information.
The PR adds this check to fix the issue of message flooding when the cluster configuration is stable

Fixes #900

[badrishc]

LGTM, please update version for a new release as well.