kernel: Move BlockInfo to a kernel file

This should avoid having to include interfaces/chain.h from a kernel
module. interfaces/chain.h in turn includes a bunch of non-kernel
headers, that break the desired library topology and might introduce
entanglement regressions.

src/index/base.cpp

@@ -9,7 +9,6 @@
 #include <dbwrapper.h>
 #include <interfaces/chain.h>
 #include <interfaces/types.h>
-#include <kernel/chain.h>
 #include <kernel/types.h>
 #include <logging.h>
 #include <node/abort.h>