Make the bounce buffer move constructible and move assignable

[kingcrimsontianyu]

Currently bounce buffer is non-copyable and non-movable. Creating a vector of bounce buffers requires the use of unique pointers as a workaround (std::vector<std::unique_ptr<Buffer>>). This PR lifts this limitation by making it move constructible and move assignable.

This PR makes the following additional improvements:

• Add unit test for the bounce buffer pool class
• Add public methods for buffer pool class used to query the state (number of free buffers held, size of the buffer) and facilitate testing
• Mark some methods const

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[kingcrimsontianyu]

Well. For a WIP PR, I no longer need the bounce buffer to be moveable. Is there any disadvantage of making it so? I'm not aware of any. But if there is, I can pivot this PR to just adding the unit test. Let me know what you think @madsbk. Thanks!

[madsbk]

Well. For a WIP PR, I no longer need the bounce buffer to be moveable. Is there any disadvantage of making it so? I'm not aware of any. But if there is, I can pivot this PR to just adding the unit test. Let me know what you think @madsbk. Thanks!

Not that I’m aware of. The buffers themselves are allocated on the heap, so making the bounce buffer movable should not make a difference. That said, we could wait with this feature until it is actually needed. I don’t have a strong opinion either way.

[vuule]

might be better to merge this now if it's ready
no negative impact AFAICT

[kingcrimsontianyu]

Thanks for the discussion.
This PR is now a dependency of #896, which uses a vector of bounce buffers to implement a k-way bounce buffer to reduce the occurrence of stream synchronization. So this can be a legitimate reason for making the bounce buffer moveable.

[madsbk]

Looks great, thanks @kingcrimsontianyu

PS: pushed a minor update to the docs

[kingcrimsontianyu]

Thanks all for your reviews.

[kingcrimsontianyu]

/merge