move-only: Move CBlockTreeDB to node/blockstorage

The block index (CBlockTreeDB) is required to write and read blocks, so
move it to blockstorage. This allows to drop the txdb.h include from
`node/blockstorage.h`.

Can be reviewed with:

--color-moved=dimmed-zebra  --color-moved-ws=ignore-all-space

src/validation.h

@@ -47,7 +47,6 @@
 #include <vector>
 
 class Chainstate;
-class CBlockTreeDB;
 class CTxMemPool;
 class ChainstateManager;
 struct ChainTxData;