PurePollution - changing the way we inject factories

Before we would create factories in the `connection` class and pass references to them when creating the `connection_impl` instance. In the long run this won't work for at least couple of reasons:
- transports will have to have a reference to the `connection_impl` to be able to let connection know about events (e.g. message received, disconnected, error etc). As a result to create a transport a (smart) pointer to the `connection_impl` will have to be passed to the `transport_factory.create()` method. However this would require making the `connection_impl type` (and all types it uses) public because the factories are members of a public (i.e. the `connection`) type. An alternative is to use uniqe pointers which will be set to default factories in the `connection_impl` itself. Tests will still be able to inject their own implementation. An additional advantage of using pointers is that now we can hide a few types that were only exposed because we had references in the connection class and since the references are gone the types don't have to exposed anymore.
- because transports will have to have a back pointer to a `connection_impl` instance it will be possible that the `connection_impl` instance will outlive the `connection` object itself (think about tasks that captured a pointer to `connection_impl` and that running after the `connection` object goes out of scope). In this scenario the transport/`connection_impl` may want to use factories while they were already destroyed because they were members of an object that has already been destroyed. This would result in undefined behavior and most likely a crash. Moving factories to the `connection_impl` fixes this issue (note: currently the `connection` type holds a raw pointer to a `connection_impl` instance which is `delete`d in the `connection`'s dtor; this will have to be changed (most likely by changing the raw pointer to a smart pointer) to prevent from trying to use an already disposed pointer in transports - same as above)
As a result of this change we are 'polluting' the code base with smart pointers but the public surface is cleaner than before.
why is it better than including headers? (As I learned only yesterday) incomplete types are tricky - for instance note that `connection_impl` does not have an explicit dtor so a compiler generated dtor will be inlined into the code that uses `connection_impl`. And because I am using `unique_ptr` and the type is incomplete the `unique_ptr` won't know how to delete the instance and since dtors were inlined no one is going to clean up and we would leak memory. Therefore I am trying not to use forward declarations in places where I don't have circular dependencies and can just include the class definition.