Fix race condition in thread initialization

The initialization of the data was not done under the same lock as thread
creation.

maxscale-cdc-adapter/src/mxs_adapter.cpp

@@ -697,6 +697,7 @@ void streamTable(std::string database, std::string table)
 
 int main(int argc, char *argv[])
 {
+    std::unique_lock<std::mutex> guard(ctx_lock);
     set_thread_id("main");
     configureSignalHandlers(signalHandler);
     config = Config::process(argc, argv);
@@ -723,13 +724,14 @@ int main(int argc, char *argv[])
     }
 
     debug("Started %lu threads", threads.size());
+    guard.unlock();
 
     // Wait until a terminate signal is received
     wait_for_signal();
 
     debug("Signal received, stopping");
 
-    ctx_lock.lock();
+    guard.lock();
 
     for (auto&& a : contexts)
     {
@@ -739,7 +741,7 @@ int main(int argc, char *argv[])
         a->cdc.close();
     }
 
-    ctx_lock.unlock();
+    guard.unlock();
 
     for (auto&& a : threads)
     {