fixed potential deadlock in CSuperblockManager::IsSuperblockTriggered (#1536)
fixed potential deadlock in CSuperblockManager::IsSuperblockTriggered (#1536)

governance.cs -> mempool.cs:
`CGovernanceManager::ProcessMessage - LOCK2(cs_main, governance.cs) ->  GetTransaction ->  lookup - LOCK(mempool.cs)`

mempool.cs -> governance.cs:
`CreateNewBlock - LOCK2(cs_main, mempool.cs)  -> FillBlockPayments ->  IsSuperblockTriggered - LOCK(governance.cs)`

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