Write witness caches when writing the best block

For steady-state operation, this reduces the average time between wallet disk
writes from once per block to once per hour.

On -rescan, witness caches are only written out at the end along with the best
block, increasing speed while ensuring that on-disk state is kept consistent.

Witness caches are now never recreated during a -reindex, on the assumption that
the blocks themselves are not changing (the chain is just being reconstructed),
and so the witnesses will remain valid.

Part of #1749.

src/primitives/block.h

@@ -200,6 +200,10 @@ struct CBlockLocator
     {
         return vHave.empty();
     }
+
+    friend bool operator==(const CBlockLocator& a, const CBlockLocator& b) {
+        return (a.vHave == b.vHave);
+    }
 };
 
 #endif // BITCOIN_PRIMITIVES_BLOCK_H