Add sleep functionality to thread wrapper.

lib/pls/include/pls/internal/base/thread.h

@@ -9,6 +9,7 @@
 #include <functional>
 #include <pthread.h>
 #include <atomic>
+#include <time.h>
 
 #include "system_details.h"
 
@@ -44,6 +45,11 @@ class this_thread {
     pthread_yield();
   }
 
+  static void sleep(long microseconds) {
+    timespec time{0, 1000 * microseconds};
+    nanosleep(&time, nullptr);
+  }
+
   /**
    * Retrieves the local state pointer.
    *