Close resource and info fork writers after upload

UploadHandler and UploadFolderHandler opened the resource and info fork
writers but only ever closed the data fork writer. On the OS-backed file
store this leaked a descriptor per uploaded file; on a backend that only
commits on Close (such as an object store, modeled by MemFileStore) the fork
data was silently dropped entirely.

Track the fork writers as closers and close them after the transfer completes,
before renaming the data fork into place. Add a MemFileStore-backed regression
test that fails without the fix.

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