Fixed some memory leaks

[Rheel17]

Fixes some memory leaks:

1. In the Linux backend, the deleter of a std::unique_ptr didn't call delete;
2. The hdr_config type and its sensor were holding mutual shared pointers to each other, causing them to not be deleted;
3. In synthetic-stream.cpp, the on_set method is called on a ptr_option, which stores the function passed to that method. However, the lambda passed captures the shared pointer, giving another shared pointer loop.

[ev-mp]

@Rheel17 , thank you for the contribution.
Actually we've been doing a parallel investigation into some of the issues highlighted, so we'll check the proposed changes and update.

[ev-mp]

@Rheel17 , it looks good.
Thank you very much indeed!