refactor, blockstorage: Replace blocksdir arg

Add a blocks_dir field to the BlockManager options. Move functions
relying on the global gArgs to get the blocks_dir into the BlockManager
class.

This should eventually allow users of the BlockManager to not rely on
the global Args and instead pass in their own options.

src/bitcoin-chainstate.cpp

@@ -89,6 +89,7 @@ int main(int argc, char* argv[])
     };
     const node::BlockManager::Options blockman_opts{
         .chainparams = chainman_opts.chainparams,
+        .blocks_dir = gArgs.GetBlocksDirPath(),
     };
     ChainstateManager chainman{chainman_opts, blockman_opts};