scheduler: don't rely on boost interrupt on shutdown

>>> adapted from bitcoin/bitcoin@306f71b

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().

Author: random-zebra

src/init.cpp

@@ -241,6 +241,7 @@ void PrepareShutdown()
 
     // After everything has been shut down, but before things get flushed, stop the
     // CScheduler/checkqueue threadGroup
+    scheduler.stop();
     threadGroup.interrupt_all();
     threadGroup.join_all();
 

src/test/test_pivx.cpp

@@ -111,6 +111,7 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
 
 TestingSetup::~TestingSetup()
 {
+        scheduler.stop();
         threadGroup.interrupt_all();
         threadGroup.join_all();
         GetMainSignals().FlushBackgroundCallbacks();