Merge #18766: Disable fee estimation in blocksonly mode (by removing the fee estimates global)

4e28753f60 feestimator: encapsulate estimation file logic (Antoine Poinsot)
e8ea6ad9c1 init: don't create a CBlockPolicyEstimator if we don't relay transactions (Antoine Poinsot)
86ff2cf202 Remove the remaining fee estimation globals (Antoine Poinsot)
03bfeee957 interface: remove unused estimateSmartFee method from node (Antoine Poinsot)

Pull request description:

  If the `blocksonly` mode is turned on after running with transaction
  relay enabled for a while, the fee estimation will serve outdated data
  to both the internal wallet and to external applications that might be
  feerate-sensitive and make use of `estimatesmartfee` (for example a
  Lightning Network node).

  This has already caused issues (for example https://github.com/bitcoin/bitcoin/issues/16840 (C-lightning), or https://github.com/lightningnetwork/lnd/issues/2562 (LND)) and it seems prudent to fail rather than to give inaccurate values.

  This fixes #16840, and closes #16890 which tried to fix the symptoms (RPC) but not the cause as mentioned by sdaftuar :
  > If this is a substantial problem, then I would think we should take action to protect our own wallet users as well (rather than hide the results of what our fee estimation would do!).

ACKs for top commit:
  MarcoFalke:
    re-ACK 4e28753f60 👋
  jnewbery:
    utACK 4e28753f60

Tree-SHA512: c869cf03b86d8194002970bbc84662dae76874967949b9be0d9a4511a1eabcb1627c38aca3154da9dcece1a4c49ec02bd4f9fcca2ec310986e07904559e63ba8

src/test/util/setup_common.cpp

@@ -16,6 +16,7 @@
 #include <net.h>
 #include <net_processing.h>
 #include <noui.h>
+#include <policy/fees.h>
 #include <pow.h>
 #include <rpc/blockchain.h>
 #include <rpc/register.h>
@@ -141,7 +142,8 @@ TestingSetup::TestingSetup(const std::string& chainName, const std::vector<const
 
     pblocktree.reset(new CBlockTreeDB(1 << 20, true));
 
-    m_node.mempool = MakeUnique<CTxMemPool>(&::feeEstimator, 1);
+    m_node.fee_estimator = std::make_unique<CBlockPolicyEstimator>();
+    m_node.mempool = std::make_unique<CTxMemPool>(m_node.fee_estimator.get(), 1);
 
     m_node.chainman = &::g_chainman;
     m_node.chainman->InitializeChainstate(*m_node.mempool);