Instead of returning a raw pointer from CZMQNotifierFactory and
implicitly requiring the caller to know that it has to take ownership,
return a std::unique_ptr to make this explicit.
This also changes the typedef for CZMQNotifierFactory to use the new
C++11 using syntax, which makes it (a little) less cryptic.
This is a backport of core#13686 [3/5]
Depends on D10283