Optimize for case where size of append dimension is one

[forman]

We can possibly avoid rolling back chunk files if the size of the target dataset's append dimension is one.

In this case, the override

zappend/zappend/rollbackstore.py

Lines 53 to 59 in c228788

	def __setitem__(self, key: str, value: bytes):
	old_value = self._store.get(key)
	self._store[key] = value
	if old_value is not None:
	self._rollback_cb("replace_file", key, old_value)
	else:
	self._rollback_cb("delete_file", key, None)

would fall back to super().__setitem__(key, value), because it is obvious that the chunk given by key does not exist.

A first step should be to verify that the statement old_value = self._store.get(key) does effectively perform a filesystem operation, e.g., stat for a local posix filesystem. If not, this optimization would not be applicable.