Separata queue for slashed events

[gusin13]

Currently both unbonding and slashed delegations events are pushed in unbonding queue, they have the same behaviour in context to stats so to avoid duplication count we use the same lock key i.e unbonding in stats.

This is bit confusing in code, so we can clear things up so each category has its own queue. We'll have to think how to avoid duplication count if both unbonding and slashed events occur for a delegation.