Serialization with "msgpack" doesn't preserve lists #3716
Description
Currently if a list is serialized (and is small enough), it will be handled correctly.
In [1]: from distributed.protocol import serialize, deserialize
In [2]: t = (0, 1, 2)
In [3]: deserialize(*serialize(t, serializers=["msgpack"]))
Out[3]: (0, 1, 2)
In [4]: l = [0, 1, 2]
In [5]: deserialize(*serialize(l, serializers=["msgpack"]))
Out[5]: [0, 1, 2]However for larger lists, this breaks down and a tuple is returned instead.
In [1]: from distributed.protocol import serialize, deserialize
In [2]: t = (0, 1, 2, 3, 4, 5, 6)
In [3]: deserialize(*serialize(t, serializers=["msgpack"]))
Out[3]: (0, 1, 2, 3, 4, 5, 6)
In [4]: l = [0, 1, 2, 3, 4, 5, 6]
In [5]: deserialize(*serialize(l, serializers=["msgpack"]))
Out[5]: (0, 1, 2, 3, 4, 5, 6)