avoid mem leaks in rs.cpp by taking ownership of callback ptrs

[maloel]

Addresses #3873 and #4447: potential memory leak in pipeline::start method (callback overloads).

Tracked on [DSO-13704]

[maloel]

I ran our sample code, and unit-tests, through clang-tidy version 12.0.1.
I failed to reproduce the errors mentioned in the above tickets.
That said, I can understand them in theory -- but am leaving until another complaint comes up as the chances of such leaks is nil.

[Nir-Az]

Looks good