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/thread.h b/lib/thread.h
index 937bedb..978aa6b 100644
--- a/lib/thread.h
+++ b/lib/thread.h
@@ -197,6 +197,7 @@
 /* Internal libzebra exports */
 extern void thread_getrusage (RUSAGE_T *);
 extern struct cmd_element show_thread_cpu_cmd;
+extern struct cmd_element clear_thread_cpu_cmd;
 
 /* replacements for the system gettimeofday(), clock_gettime() and
  * time() functions, providing support for non-decrementing clock on