Add ats_unique_buf

[maskit]

Add a mechanism to use std::unique_ptr with ats_malloc.

This is required by QUIC code.

[randall]

[approve ci autest]

[zizhong]

LGTM

[SolidWallOfCode]

Any reason to not use std::vector<uint8_t>?

[maskit]

@SolidWallOfCode I don't see any reasons to use std::vector<uint8_t>. What I want to do is managing buffer allocated by ats_malloc with a smart pointer mechanism. Are you suggesting we don't need to use ats_malloc?

[maskit]

Talked on Slack, and my understanding is:
We could/should use std::vector<uint8_t> or std::string instead of ats_malloc in general. Because these two should provide what we need, we might be able to deprecate ats_malloc in the future.

For now, I'd like to stick with the ordinary way (ats_malloc). Replacing internal implementation later shouldn't be difficult.

Merging this for coming 9.0 branching.