Add option to forceReconnect API to guarantee fair server distribution

[ajax-koval-d]

This addresses #1184

[ajax-koval-d]

Did not cover all possible combinations since in that case locally test was running 17 secs and almost never passed

[scottf]

@ajax-koval-d I'm not going to accept the PR for the following reasons.

1. The NatsServerPool implementation matches the behavior in the other clients. Adding behavior that is not standard is confusing to developers who use multiple language clients.
2. There is already a way to provide your own implementation of ServerPool
3. The idea behind force reconnect is not to reconnect to the same server. It's used in error conditions, lame duck mode handling and when you specifically want to disconnect from the current server. Reconnecting to the same server defeats the purpose.
4. For backward compatibility, all new api on existing interfaces must have a default implementation, otherwise it is a breaking change.

[scottf]

@ajax-koval-d I am willing to discuss changes to the ServerPool interface that allows signaling of state, giving more flexibility to behavior of ServerPool implementations. For instance shuffle would not make sense, but something that indicates a connection state about to change/changed.

[ajax-koval-d]

@scottf Regarding point 3 - my impression was, that according to this adr one of the purposes of reconnect api is also rebalance, and current implementation does not always fulfill that.

My problem with implementing ServerPool is that I don't need to modify usual reconnect flow, all I want to change is force reconnect behaviour.

something that indicates a connection state about change/to change

Not sure that I follow you here. Do you mean to provide new method in ServerPool interface, like goingToForceReconnect so that there'll be ability to react on it in implementation?