Remove unused includes from txmempool.h

... and move them to where they are really needed.

This was found by IWYU:

txmempool.h should remove these lines:
- #include <random.h>  // lines 29-29
- class CBlockIndex;  // lines 43-43
- class Chainstate;  // lines 45-45

Also, move the stdlib section to the right place. Can be reviewed with:
--color-moved=dimmed-zebra

src/node/miner.h

@@ -21,10 +21,11 @@
 #include <boost/multi_index_container.hpp>
 
 class ArgsManager;
-class ChainstateManager;
 class CBlockIndex;
 class CChainParams;
 class CScript;
+class Chainstate;
+class ChainstateManager;
 
 namespace Consensus { struct Params; };
 

src/test/miniminer_tests.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 #include <node/mini_miner.h>
+#include <random.h>
 #include <txmempool.h>
 #include <util/time.h>
 

src/txmempool.h

@@ -6,15 +6,6 @@
 #ifndef BITCOIN_TXMEMPOOL_H
 #define BITCOIN_TXMEMPOOL_H
 
-#include <atomic>
-#include <map>
-#include <optional>
-#include <set>
-#include <string>
-#include <string_view>
-#include <utility>
-#include <vector>
-
 #include <kernel/mempool_limits.h>
 #include <kernel/mempool_options.h>
 
@@ -26,7 +17,6 @@
 #include <policy/feerate.h>
 #include <policy/packages.h>
 #include <primitives/transaction.h>
-#include <random.h>
 #include <sync.h>
 #include <util/epochguard.h>
 #include <util/hasher.h>
@@ -40,9 +30,16 @@
 #include <boost/multi_index/tag.hpp>
 #include <boost/multi_index_container.hpp>
 
-class CBlockIndex;
+#include <atomic>
+#include <map>
+#include <optional>
+#include <set>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
 class CChain;
-class Chainstate;
 
 /** Fake height value used in Coin to signify they are only in the memory pool (since 0.8) */
 static const uint32_t MEMPOOL_HEIGHT = 0x7FFFFFFF;