Create more informative configuration documentation.

[djandrewd]

1. ClusterConfig contains configuration without javadoc and therefore without source code it is impossible to get meaning of most of configuration.
2. Add TimeUnit to all duration entries. It is hard to understand that they are in ms.
3. Add configurable constant gossip period.

[antonkharenko]

@djandrewd Hi, thanks for valuable feedback. I will definitely do it, but I am curious if you can share a bit on a need and use case for the configurable gossip period? And just to clarify there is a property gossipInterval which defines a time between sending gossips and it is configurable, but also there is a computable property ClusterMath.gossipPeriodsToSpread(...) and I assume that you are speaking about the second one.

[djandrewd]

Yes, I was talking about ClusterMath.gossipPeriodsToSpread. I need to spread gossip for each of the node only once for example.