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

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