scmutil: support background file closing Closing files that have been appended to is relatively slow on Windows/NTFS. This makes several Mercurial operations slower on Windows. The workaround to this issue is conceptually simple: use multiple threads for I/O. Unfortunately, Python doesn't scale well to multiple threads because of the GIL. And, refactoring our code to use threads everywhere would be a huge undertaking. So, we decide to tackle this problem by starting small: establishing a thread pool for closing files. This patch establishes a mechanism for closing file handles on separate threads. The coordinator object is basically a queue of file handles to operate on and a thread pool consuming from the queue. When files are opened through the VFS layer, the caller can specify that delay closing is allowed. A proxy class for file handles has been added. We must use a proxy because it isn't possible to modify __class__ on built-in types. This adds some overhead. But as future patches will show, this overhead is cancelled out by the benefit of closing file handles on background threads.