Drop Pool job_sender explicitly using ManuallyDrop

[flpcury]

Closing the channel is what allows the threads to shut down gracefully. Therefore, we want to drop the job_sender (closing the channel) before the threads are joined.

That was already being accomplished by having job_sender as an Option, and setting it to None in the destructor, which caused it to be explicitly dropped before the rest of the struct.

The std::mem::ManuallyDrop wrapper, stabilized in Rust v1.20.0, was introduced to be a more performant and ergonomic way to explicitly drop in an arbitrary order.

Alternatively, the correct drop order could also be accomplished implicitly, by changing the order in which the Pool struct fields are declared. However, the previous solution was an explicit one, so this change preserves that.