Expose open/close for connection pooling

[dminuoso]

This change makes the necessary changes to the library to support pooling of resources.

Using resource-pool one can now do the following:

withLdap' :: Pool LdapH
          -> (Ldap -> IO a)
          -> IO a
withLdap' pool act = do
  (ldapH, local) <- takeResource pool
  res <- runsInEither act ldapH `E.onException` destroyResource pool local ldapH
  case res of
    Right a -> putResource local ldapH *> pure a
    Left e -> destroyResource pool local ldapH *> throwIO e

Or if you want to do this with unliftio you could do:

withLdap :: MonadUnliftIO m
         => Pool LdapH
         -> (Ldap -> m a)
         -> m a
withLdap pool act = withRunInIO $ \io -> do
  (ldapH, local) <- liftIO (takeResource pool)
  res <- liftIO (runsInEither (io . act) ldapH) `onException` liftIO (destroyResource pool local ldapH)
  case res of
    Right a -> liftIO (putResource local ldapH) *> pure a
    Left e  -> liftIO (destroyResource pool local ldapH) *> throwIO e

The change is backwards compatible.

Along the changes this also now properly catches async exceptions send to one of the worker threads.

[dminuoso]

This fixes #2 and #3

[dminuoso]

Test suite now finishes.

I've taken the liberty to make some minor refactoring as well.