add ValidationInterface::ActiveTipChange

This is a synchronous callback notifying clients of all tip changes.

It allows clients to respond to a new block immediately after it is
connected. The synchronicity is important for things like
m_recent_rejects, in which a transaction's validity can change (rejected
vs accepted) when this event is processed. For example, the transaction
might have a timelock condition that has just been met. This is distinct
from something like m_recent_confirmed_transactions, in which the
validation outcome is the same (valid vs already-have), so it does not
need to be reset immediately.

src/validationinterface.h

@@ -61,6 +61,10 @@ protected:
      * Called on a background thread. Only called for the active chainstate.
      */
     virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
+    /**
+     * Notifies listeners any time the block chain tip changes, synchronously.
+     */
+    virtual void ActiveTipChange(const CBlockIndex* new_tip, bool is_ibd) {};
     /**
      * Notifies listeners of a transaction having been added to mempool.
      *
@@ -214,6 +218,7 @@ public:
     void SyncWithValidationInterfaceQueue() LOCKS_EXCLUDED(cs_main);
 
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
+    void ActiveTipChange(const CBlockIndex*, bool);
     void TransactionAddedToMempool(const NewMempoolTransactionInfo&, uint64_t mempool_sequence);
     void TransactionRemovedFromMempool(const CTransactionRef&, MemPoolRemovalReason, uint64_t mempool_sequence);
     void MempoolTransactionsRemovedForBlock(const std::vector<RemovedMempoolTransactionInfo>&, unsigned int nBlockHeight);