optional_api: Refactor to use vector's and standard allocators.

* Replace ad-hoc array management with macro's from vector.h.
* Remove redundent logger messages.
* Use normal Asterisk allocators instead of directly using libc
  allocators.
* Free memory when an API has no implementation or users.

Change-Id: Ic6ecb31798d4a78e7df39ece86a68b60eac05bf5