src: use thread defaults provided by v8::platform

By passing const int thread_pool_size = 0 to CreateDefaultPlatform we allow v8 to choose sensible defaults based on the currently online processors.

Benchmarking on my local machine saw no performance regressions; however it is dual core and hyper-threaded and would of used the same thread count as previously - I assume that this will benefit machines with more online processors; however I am curious if this would affect machines with only one core.