ARROW-2539: [Plasma] Use unique_ptr instead of raw pointer

[zhijunfu]

use unique_ptr to replace raw pointer, so that allocated memory can be freed automatically

[xhochy]

Instead of using uint8_t[] we could also use std::vector<uint8_t> to better track the memory.

[zhijunfu]

@xhochy Thanks for the comment. I checked current plasma code and it uses unique_ptr<uint8_t[]> for similar cases when wrapping a raw pointer, it seems that using unique_ptr is more consistent with the rest of code compared to using a vector.

[pcmoritz]

Thanks for doing this, it is definitely an improvement! I'm going to merge and then we can see if it is possible to get rid of this and the other uint8_t* and replace them by vectors in the rest of the codebase.

[zhijunfu]

Thanks Philipp:)

[pcmoritz]

@zhijunfu Do you have an idea if this issue could be related to the change: https://issues.apache.org/jira/browse/ARROW-2552

It could also be 0f07171 or the combination of the two (?)