Clean up the "state" to indicate that there is no longer any
asynchronous operation. Fixes another NFS-related crash due to
cleanup of a non-existing asynchronous operation.
Creating a NfsStorage sets its own export_name as the "base". Now
NfsFileReader can use this information to derive the export_name to be
mounted, instead of guessing. This solves the "too many connection"
problem on the NFS server while updating the database.
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().
