Automatically discard task result objects once received by the client

[rafa-be]

Currently, results data is kept by the scheduler until the client it's associated disconnects.

Ideally, we could discard the task result once received by the client:

In [1]: import math

In [2]: from scaler import Client

In [3]: client = Client("tcp://127.0.0.1:60338")

# ScalerTop shows 1 scheduler object (the serializer)

In [4]: future = client.submit(math.cos, 0)

# ScalerTop shows 4 scheduler objects (the serializer, the function, the function argument and the return value).

In [5]: future.result()
Out[5]: 1.0

# ScalerTop still shows 4 objects, while the scheduler could clear the task's result data as it reached the client's future. 

In [6]: client.disconnect()

# ScalerTop shows 0 scheduler object

[rafa-be]

Note that the client cannot always send a delete request when receiving the result object, as this result object might be required by another dependent graph task.