node: Add reference to mempool in NodeContext

Currently it is an alias to the global ::mempool and should be used as
follows.

* Node code (validation and transaction relay) can use either ::mempool
  or node.mempool, whichever seems a better fit.
* RPC code should use the added convenience getter EnsureMempool, which
  makes sure the mempool exists before use. This prepares the RPC code
  to a future where the mempool might be disabled at runtime or compile
  time.
* Test code should use m_node.mempool directly, as the mempool is always
  initialized for tests.

src/test/util/setup_common.cpp

@@ -107,7 +107,6 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
     threadGroup.create_thread(std::bind(&CScheduler::serviceQueue, &scheduler));
     GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
 
-    mempool.setSanityCheck(1.0);
     pblocktree.reset(new CBlockTreeDB(1 << 20, true));
     g_chainstate = MakeUnique<CChainState>();
     ::ChainstateActive().InitCoinsDB(
@@ -131,6 +130,8 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
     }
     g_parallel_script_checks = true;
 
+    m_node.mempool = &::mempool;
+    m_node.mempool->setSanityCheck(1.0);
     m_node.banman = MakeUnique<BanMan>(GetDataDir() / "banlist.dat", nullptr, DEFAULT_MISBEHAVING_BANTIME);
     m_node.connman = MakeUnique<CConnman>(0x1337, 0x1337); // Deterministic randomness for tests.
 }
@@ -144,6 +145,7 @@ TestingSetup::~TestingSetup()
     g_rpc_node = nullptr;
     m_node.connman.reset();
     m_node.banman.reset();
+    m_node.mempool = nullptr;
     UnloadBlockIndex();
     g_chainstate.reset();
     pblocktree.reset();