JSON adapter needs this

Adapter.cc

@@ -42,6 +42,14 @@ namespace pEp {
         ::utility::locked_queue< SYNC_EVENT, ::free_Sync_event > q;
         std::mutex m;
 
+        std::thread::id sync_thread_id()
+        {
+            if (_sync_thread)
+                return _sync_thread->get_id();
+            else
+                return std::thread::id();
+        }
+
         int _inject_sync_event(SYNC_EVENT ev, void *management)
         {
             try {

Adapter.hh

@@ -5,8 +5,9 @@
 
 #include <functional>
 #include <string>
-#include <pEp/sync_api.h>
+#include <thread>
 #include <stdexcept>
+#include <pEp/sync_api.h>
 
 namespace pEp {
 
@@ -36,6 +37,9 @@ namespace pEp {
         // returns 'true' when called from the "sync" thread, 'false' otherwise.
         bool on_sync_thread();
 
+        // returns the thread id of the sync thread
+        std::thread::id sync_thread_id();
+
         enum session_action {
             init,
             release