ping: use a monotonic timer

The deadline_timer uses system's UTC clock which is not monotonic (i.e.,
it jumps when the wall clock is adjusted). Let's switch to a monotonic
timer.

Given that boost::asio::steady_timer::expires_from_now() is marked as
deprecated, use the other function.

Change-Id: I40383721ecb0f12bfcb3f638124a32150a29bf48

Author: jktjkt

src/http/EventStream.cpp

@@ -180,7 +180,7 @@ void EventStream::enqueue(const std::string& fieldName, const std::string& what)
 
 void EventStream::start_ping()
 {
-    ping.expires_from_now(boost::posix_time::seconds(m_keepAlivePingInterval.count()));
+    ping.expires_after(m_keepAlivePingInterval);
     ping.async_wait([weak = weak_from_this()](const boost::system::error_code& ec) {
         auto myself = weak.lock();
         if (!myself) {

src/http/EventStream.h

@@ -7,7 +7,7 @@
 
 #pragma once
 
-#include <boost/asio/deadline_timer.hpp>
+#include <boost/asio/steady_timer.hpp>
 #include <boost/signals2.hpp>
 #include <list>
 #include <memory>
@@ -50,7 +50,7 @@ class EventStream : public std::enable_shared_from_this<EventStream> {
     };
 
     State state = WaitingForEvents;
-    boost::asio::deadline_timer ping;
+    boost::asio::steady_timer ping;
     std::list<std::string> queue;
     mutable std::mutex mtx; // for `state` and `queue`
     boost::signals2::scoped_connection eventSub, terminateSub;