Small improvement: using lock_guard instead of unique_lock

[hyperxor]

Hello,

I want to propose small improvement: using std::lock_guard instead of std::unique_lock. The size of std::unique_lock is 16 bytes, the size of std::lock_guard is 8 bytes. We can change using std::unique_lock without any problems because locks are trivial.

[p-ranav]

Thanks!