scheduler: don't rely on boost interrupt on shutdown

Calling interrupt_all() will immediately stop the scheduler, so it's
safe to invoke stop() beforehand, and this removes the reliance on boost
to interrupt serviceQueue().

src/init.cpp

@@ -206,6 +206,7 @@ void Shutdown(NodeContext& node)
 
     // After everything has been shut down, but before things get flushed, stop the
     // CScheduler/checkqueue threadGroup
+    if (node.scheduler) node.scheduler->stop();
     threadGroup.interrupt_all();
     threadGroup.join_all();
 

src/test/txindex_tests.cpp

@@ -70,6 +70,8 @@ BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
     // shutdown sequence (c.f. Shutdown() in init.cpp)
     txindex.Stop();
 
+    // txindex job may be scheduled, so stop scheduler before destructing
+    m_node.scheduler->stop();
     threadGroup.interrupt_all();
     threadGroup.join_all();
 

src/test/util/setup_common.cpp

@@ -140,6 +140,7 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
 
 TestingSetup::~TestingSetup()
 {
+    if (m_node.scheduler) m_node.scheduler->stop();
     threadGroup.interrupt_all();
     threadGroup.join_all();
     GetMainSignals().FlushBackgroundCallbacks();