BUILD: tree-wide: Avoid warnings about undefined entities retrieved from a CS

BUILD: tree-wide: Avoid warnings about undefined entities retrieved from a CS

Since recent changes related to the conn-stream/stream-interface
refactoring, GCC reports potential null pointer dereferences when we get the
appctx, the stream or the stream-interface from the conn-strem. Of course,
depending on the time, these entities may be null. But at many places, we
know they are defined and it is safe to get them without any check. Thus, we
use ALREADY_CHECKED() macro to silent these warnings.

Note that the refactoring is unfinished, so it is not a real issue for now.