fix a deadlock or sporadic crash during shutdown

src/com/ZmqContext.cpp

@@ -13,8 +13,23 @@
 
 using namespace aman;
 
-static zmq::context_t __context(2);
+ZmqContext::ZmqContext() : m_context() { }
+
+void ZmqContext::initialize() {
+    if (nullptr == this->m_context)
+        this->m_context = std::make_unique<zmq::context_t>(2);
+}
+
+void ZmqContext::deinitialize() {
+    if (nullptr != this->m_context)
+        this->m_context = std::unique_ptr<zmq::context_t>();
+}
 
 zmq::context_t& ZmqContext::context() {
-    return __context;
+    return *this->m_context;
+}
+
+ZmqContext& ZmqContext::instance() {
+    static ZmqContext __instance;
+    return __instance;
 }