Merge bitcoin/bitcoin#27460: rpc: Add importmempool RPC

fa776e61cd Add importmempool RPC (MarcoFalke)
fa20d734a2 refactor: Add and use kernel::ImportMempoolOptions (MarcoFalke)
fa8866990d doc: Clarify the getmempoolinfo.loaded RPC field documentation (MarcoFalke)
6888886cec Remove Chainstate::LoadMempool (MarcoFalke)

Pull request description:

  Currently it is possible to import a mempool by placing it in the datadir and starting the node. However this has many issues:

  * Users aren't expected to fiddle with the datadir, possibly corrupting it
  * An existing mempool file in the datadir may be overwritten
  * The node needs to be restarted
  * Importing an untrusted file this way is dangerous, because it can corrupt the mempool

  Fix all issues by adding a new RPC.

ACKs for top commit:
  ajtowns:
    utACK fa776e61cd
  achow101:
    ACK fa776e61cd
  glozow:
    reACK fa776e61cd

Tree-SHA512: fcb1a92d6460839283c546c47a2d930c363ac1013c4c50dc5215ddf9fe5e51921d23fe0abfae0a5a7631983cfc7e2fff3788b70f95937d0a989a203be4d67546

src/init.cpp

@@ -116,6 +116,7 @@
 #endif
 
 using kernel::DumpMempool;
+using kernel::LoadMempool;
 using kernel::ValidationCacheSizes;
 
 using node::ApplyArgsManOptions;
@@ -1676,7 +1677,10 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
             return;
         }
         // Load mempool from disk
-        chainman.ActiveChainstate().LoadMempool(ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{});
+        if (auto* pool{chainman.ActiveChainstate().GetMempool()}) {
+            LoadMempool(*pool, ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{}, chainman.ActiveChainstate(), {});
+            pool->SetLoadTried(!chainman.m_interrupt);
+        }
     });
 
     // Wait for genesis block to be processed