Overhaul of thread handling to support blocking functions other than wait

luprex/core/cpp/sched.cpp

@@ -24,6 +24,8 @@ eng::string SchedEntry::debug_string() const {
 }
 
 void Schedule::add(int64_t clk, int64_t thid, int64_t plid) {
+    assert(plid != 0);
+    assert(thid != 0);
     schedule_.insert(SchedEntry(clk, thid, plid));
 }