msg/async: refine worker creation in NetworkStack

msg/async: refine worker creation in NetworkStack

This commit updates NetworkStack::create_worker() to a virtual
function to reduce type check redundancy.

Since calling a pure virtual function in a constructor method is
strictly prohibited, NetworkStack::create_worker() is currently
not implemented as a virtual function.
It requires duplicated type check with NetworkStack::create(),
making the code a little bit tricky.

Considering NetworkStack instances can only be instantiated
through NetworkStack::create(), this commit moves worker creation
out of its constructor to NetworkStack::create(), makes it a pure
virtual, and lets inherited classes implement it to remove the
type check redundancy. By making it a non-static, we can also
reduce unnecessary class member function exposure.

Signed-off-by: Insu Jang <insu_jang@tmax.co.kr>