validation: indexing changes for assumeutxo

When using an assumedvalid chainstate, only process validationinterface
callbacks from the background chainstate within indexes. This ensures
that all indexes are built in-order.

Later, we can possibly designate indexes which can be built out of order
and continue their operation during snapshot use.

Once the background sync has completed, restart the indexes so that
they continue to index the now-validated snapshot chainstate.

src/validation.cpp

@@ -3289,6 +3289,16 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
 
         if (WITH_LOCK(::cs_main, return m_disabled)) {
             // Background chainstate has reached the snapshot base block, so exit.
+
+            // Restart indexes to resume indexing for all blocks unique to the snapshot
+            // chain. This resumes indexing "in order" from where the indexing on the
+            // background validation chain left off.
+            //
+            // This cannot be done while holding cs_main (within
+            // MaybeCompleteSnapshotValidation) or a cs_main deadlock will occur.
+            if (m_chainman.restart_indexes) {
+                m_chainman.restart_indexes();
+            }
             break;
         }
 
@@ -5921,3 +5931,11 @@ bool ChainstateManager::ValidatedSnapshotCleanup()
     }
     return true;
 }
+
+Chainstate& ChainstateManager::GetChainstateForIndexing()
+{
+    // We can't always return `m_ibd_chainstate` because after background validation
+    // has completed, `m_snapshot_chainstate == m_active_chainstate`, but it can be
+    // indexed.
+    return (this->GetAll().size() > 1) ? *m_ibd_chainstate : *m_active_chainstate;
+}