fix FileDownloadDelegate

[weissi]

FileDownloadDelegate calls pool.start() on a NIOThreadPool that is either provided by the user or by AHC itself through

public init(
        path: String,
        pool: NIOThreadPool = NIOThreadPool(numberOfThreads: 1),
        reportHead: ((HTTPResponseHead) -> Void)? = nil,
        reportProgress: ((Progress) -> Void)? = nil
    )

AHC however never calls .shutdownGracefully/.syncShutdownGracefully on that pool. That means the user has to provide a NIOThreadPool, or else we'll just leak it (because AHC never calls stop).

Ie. in the default use case we

• spawn a fresh thread per download
• and then leak the thread

😭