Refactor tpool::cache

While analysing MDEV-29190, marko voiced concerns over tpool::cache thread safety, mainly over the use of std::vector methods push_back(), pop_back(), and size()

It was not apparent by reading the code that the cache does not grow, and the vector is not getting reallocated (this does not happen, because push_back() should only be applied to prevously pop_back()ed element)

Also, there was a complaint that it is hard to impossible to debug the elements that were "borrowed" from cache, and there is not much comments for the class.