Add batch sampling support

[sile]

[contramundum53]

How is this different from sampling one at a time?

[sile]

In batch sampling mode, solvers would receive multiple ask requests before receiving the corresponding evaluation results (tell) as follows (assumes the batch size is 3):

1. ask
2. ask (samplers need to decide next parameters without knowing the evaluation result of the previous ask)
3. ask (ditto)
4. tell
5. tell
6. tell

On the other hand, in "one at a time", the ask and tell is interleaved:

1. ask
2. tell
3. ask
4. tell
5. ask
6. tell