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.cpp

@@ -183,6 +183,12 @@ void ValidationSignals::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlo
                           fInitialDownload);
 }
 
+void ValidationSignals::ActiveTipChange(const CBlockIndex *new_tip, bool is_ibd)
+{
+    LOG_EVENT("%s: new block hash=%s block height=%d", __func__, new_tip->GetBlockHash().ToString(), new_tip->nHeight);
+    m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.ActiveTipChange(new_tip, is_ibd); });
+}
+
 void ValidationSignals::TransactionAddedToMempool(const NewMempoolTransactionInfo& tx, uint64_t mempool_sequence)
 {
     auto event = [tx, mempool_sequence, this] {