nfs_destroy_context() will invoke all pending callbacks with
err==-EINTR. In CancellableCallback::Callback(), this will invoke
NfsConnection::DeferClose(), which however is only designed to be
called from nfs_service(). In non-debug mode, this will leak memory
because nfs_close_async() is never called.
Workaround: before nfs_destroy_context(), invoke nfs_close_async() on
all pending file handles.
If an async opertion is in progress, nfs_close_async() will make
libnfs crash because the RPC callback will dereference an object that
was freed by nfs_close_async().
