Don't recommend `forkIO`

[NorfairKing]

The docs for pubSubForever use forkIO:

hedis/src/Database/Redis/PubSub.hs

Line 548 in f3ce898

-- forkIO $ forever $

but that's probably not what we want to be using.
For example, if the forked thread crashes, the other threads will not notice.
Could we recommend using withAsync instead?

See the docs here for more details on both the problem and the solution: http://hackage.haskell.org/package/async-2.2.3/docs/Control-Concurrent-Async.html

cc @vidocco

[NorfairKing]

Here's another example just in case:

#!/usr/bin/env stack
-- stack --resolver lts-15.15 script
{-# LANGUAGE NumericUnderscores #-}
import Control.Concurrent
main :: IO ()
main = do
  putStrLn "Starting our 'server'."
  forkIO $ do            
    putStrLn "Serving..."
    threadDelay 1_000_000
    putStrLn "Oh no, about to crash!"
    threadDelay 1_000_000
    putStrLn "Aaaargh"
    undefined
  threadDelay 5_000_000
  putStrLn "Still running, eventhough we crashed"
  threadDelay 5_000_000                 
  putStrLn "Ok that's enough of that, stopping here."

Which outputs:

$ ./test.hs
Starting our 'server'.
Serving...
Oh no, about to crash!
Aaaargh
test.hs: Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:80:14 in base:GHC.Err
  undefined, called at /home/syd/test/test.hs:17:5 in main:Main
Still running, eventhough we crashed
Ok that's enough of that, stopping here.

[k-bx]

Yes, this sounds great. Would happily accept a PR on this one.