Semantic/format and queue emplace improvement.

I'm purposing a semantic/format fix to fix some inconsistency where some parts of the code would refer to the members via "this->" and others reference the member variables directly. I've changed all to "this->mem" format. Some brackets are moved to be more consistent over all. I've also added void to signatures for readability. I realise this is just personally preference and you might not like it, so it's okay to not accept this.

I've also changed push to emplace in the enqueue method. Since we are constructing a std::function with a lambda, I think emplace makes sense here.

I also think it would be appropriate to have a default constructor as below:
> ThreadPool(void)
> : ThreadPool(std::thread::hardware_concurrency() > 0
> ? std::thread::hardware_concurrency() : 2) {}

It would look better if we have a static local function
> static size_t getDefaultThreadPoolSize(void)
> {
>     const auto size = std::thread::hardware_concurrency();
>     return size > 0 ? size : 2; // or another suitable number.
> }

then we can have:
> ThreadPool(void) : ThreadPool(getDefaultThreadPoolSize()) {}


Let me know what you think.