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/qt/test/rpcnestedtests.cpp

@@ -32,7 +32,6 @@ void RPCNestedTests::rpcNestedTests()
     // do some test setup
     // could be moved to a more generic place when we add more tests on QT level
     tableRPC.appendCommand("rpcNestedTest", &vRPCCommands[0]);
-    //mempool.setSanityCheck(1.0);
 
     TestingSetup test;