Revise cpp codes (mutex/cond)

Revise cpp codes (mutex/cond)

use std::mutex instead of pthread_mutex for internal locking.
use lock_guard/unique_lock whenever possible, which is convenient RAII-style mechanism for owning a mutex.
use std::condition_variable instead of pthread_cond for internal wait/signal

[Version] 0.5.26
[Issue Type] Revise

Change-Id: I2aee6405c173c5df2b391998d8e483e1c528c453