Make Watchdog use single thread dispatch (#1347)

Notifier has one thread per instance because the callbacks must be
asynchronous. Watchdog callbacks can be synchronous, so this overhead
can be done away with via a scheduler thread akin to what the HAL
Notifier does.

wpiutil/src/main/native/include/wpi/PriorityQueue.h

@@ -0,0 +1,40 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_WPI_PRIORITYQUEUE_H_
+#define WPIUTIL_WPI_PRIORITYQUEUE_H_
+
+#include <algorithm>
+#include <functional>
+#include <queue>
+#include <vector>
+
+namespace wpi {
+
+/**
+ * This class adds a method for removing all elements from the priority queue
+ * matching the given value.
+ */
+template <class T, class Container = std::vector<T>,
+          class Compare = std::less<typename Container::value_type>>
+class PriorityQueue : public std::priority_queue<T, Container, Compare> {
+ public:
+  bool remove(const T& value) {
+    auto it = std::find(this->c.begin(), this->c.end(), value);
+    if (it != this->c.end()) {
+      this->c.erase(it);
+      std::make_heap(this->c.begin(), this->c.end(), this->comp);
+      return true;
+    } else {
+      return false;
+    }
+  }
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_PRIORITYQUEUE_H_