refactor: deduplicate host resolution logic and observations

Replaces the "API server listening on unix socket" with simpler
"API server listening on " observation. This allows refactoring
redundant code.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

test/doc/Main.hs

@@ -18,6 +18,7 @@ main =
     , "src/PostgREST/Config.hs"
     , "src/PostgREST/Error.hs"
     , "src/PostgREST/MediaType.hs"
+    , "src/PostgREST/Network.hs"
     , "src/PostgREST/Plan.hs"
     , "src/PostgREST/Query/SqlFragment.hs"
     , "src/PostgREST/Response.hs"