lib: Add a command to clear the thread CPU history data

* (general) this can be useful when investigating thread latency problems,
  when you don't want to have to restart a daemon between tests.
* thread.c: (cpu_record_(hash_)clear) wipe the stored thread cpu history
  data, according to the filter, similar to the vty print code.
  (clear_thread_cpu_cmd) new command to clear data.
* thread.h: export new command
* command.c: install it
diff --git a/lib/command.c b/lib/command.c
index 478125f..5a13f39 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -3650,6 +3650,8 @@
       install_element (VIEW_NODE, &show_thread_cpu_cmd);
       install_element (ENABLE_NODE, &show_thread_cpu_cmd);
       install_element (RESTRICTED_NODE, &show_thread_cpu_cmd);
+      
+      install_element (ENABLE_NODE, &clear_thread_cpu_cmd);
       install_element (VIEW_NODE, &show_work_queues_cmd);
       install_element (ENABLE_NODE, &show_work_queues_cmd);
     }