Handle orphan chain states from left subnets.

[adlrocha]

In the current implementation of the system, when a peer leaves a subnet, its chain state is not removed from local storage. We need to decide how we want to handle this. We can either:

• Add a flag to let the user choose if it wants to remove the state of a subnet or not. This would require some logic to inform the user about the state for which chains it still keeps.
• Always remove the state for a subnet we are no longer part of.
• Only remove the state of a subnet when it's been killed.

(Probably the first option is the most flexible, that way we delegate the decision to end-users according to their needs).

[adlrocha]

This issue is is similar to #54 and may depend on it