kernel: Remove some unnecessary non-kernel includes

Specifically gets rid of batchpriority, chainparams, script/sign.h and
system includes.

Also take the opportunity of cleaning up the headers for the effected
files and adding them to the iwyu-enforced set.

src/node/utxo_snapshot.cpp

@@ -8,7 +8,6 @@
 #include <streams.h>
 #include <sync.h>
 #include <tinyformat.h>
-#include <txdb.h>
 #include <uint256.h>
 #include <util/fs.h>
 #include <validation.h>
@@ -16,6 +15,7 @@
 #include <cassert>
 #include <cstdio>
 #include <optional>
+#include <span>
 #include <string>
 
 namespace node {