backport: Use std::unique_ptr (C++11) where possible (bitcoin #11043)

[fdoving]

Original-author: practicalswift

Use std::unique_ptr (C++11) where possible.

Rationale:

Avoid resource leaks (specifically: forgetting to delete an object created using new) Avoid undefined behaviour (specifically: double delete:s)

Changes: Resolving conflicts and adjustments for Ravencoin and assets.

[fdoving]

TODO: Assets still need some work.

[fdoving]

Re-added the assert.