correct exponential backoff on Listener

Clears the limitation mentioned on

https://github.com/PostgREST/postgrest/pull/3536

The Listener no longer uses the https://hackage.haskell.org/package/retry
package and instead uses a much simpler IORef in AppState for the
delays.

Additionally it no longer uses exception throwing/catching, which
is rather messy and brings some
concerns(https://github.com/PostgREST/postgrest/issues/3569#issuecomment-2146013327).

src/PostgREST/App.hs

@@ -120,7 +120,7 @@ postgrest logLevel appState connWorker =
         -- the connWorker is done.
         when (isServiceUnavailable response) connWorker
         resp <- do
-          delay <- AppState.getRetryNextIn appState
+          delay <- AppState.getNextDelay appState
           return $ addRetryHint delay response
         respond resp