make BlockUntilSyncedToCurrentChain() const

The method checks the chain tip for the best block, and calls SyncWithValidationInterfaceQueue() (a standalone function) if necessary.

src/index/base.cpp

@@ -270,7 +270,7 @@ void BaseIndex::ChainStateFlushed(const CBlockLocator& locator)
     Commit();
 }
 
-bool BaseIndex::BlockUntilSyncedToCurrentChain()
+bool BaseIndex::BlockUntilSyncedToCurrentChain() const
 {
     AssertLockNotHeld(cs_main);
 

src/index/base.h

@@ -97,7 +97,7 @@ public:
     /// sync once and only needs to process blocks in the ValidationInterface
     /// queue. If the index is catching up from far behind, this method does
     /// not block and immediately returns false.
-    bool BlockUntilSyncedToCurrentChain();
+    bool BlockUntilSyncedToCurrentChain() const;
 
     void Interrupt();