ShardedDeviceArray access should be threadsafe

[skye]

This used to be the case, but I broke it in 07571ae here: https://github.com/google/jax/blob/master/jax/interpreters/pxla.py#L499 (self._npy_value may be accessed by another thread before it's populated). I'm planning to fix this today or Monday.