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/test/util/setup_common.cpp

@@ -63,6 +63,7 @@
 #include <functional>
 #include <stdexcept>
 
+using kernel::CBlockTreeDB;
 using kernel::ValidationCacheSizes;
 using node::ApplyArgsManOptions;
 using node::BlockAssembler;