Compute the mapping of server (_Host instance) -> list of keys to stuff onto that server, as well as the mapping of
        prefixed key -> original key.

def _map_and_prefix_keys(self, key_iterable, key_prefix):
        """Compute the mapping of server (_Host instance) -> list of keys to stuff onto that server, as well as the mapping of
        prefixed key -> original key.


        """
        # Check it just once ...
        key_extra_len=len(key_prefix)
        if key_prefix and self.do_check_key:
            self.check_key(key_prefix)

        # server (_Host) -> list of unprefixed server keys in mapping
        server_keys = {}

        prefixed_to_orig_key = {}
        # build up a list for each server of all the keys we want.
        for orig_key in key_iterable:
            if isinstance(orig_key, tuple):
                # Tuple of hashvalue, key ala _get_server(). Caller is essentially telling us what server to stuff this on.
                # Ensure call to _get_server gets a Tuple as well.
                str_orig_key = str(orig_key[1])
                server, key = self._get_server((orig_key[0], key_prefix + str_orig_key)) # Gotta pre-mangle key before hashing to a server. Returns the mangled key.
            else:
                str_orig_key = str(orig_key) # set_multi supports int / long keys.
                server, key = self._get_server(key_prefix + str_orig_key)

            # Now check to make sure key length is proper ...
            if self.do_check_key:
                self.check_key(str_orig_key, key_extra_len=key_extra_len)

            if not server:
                continue

            if server not in server_keys:
                server_keys[server] = []
            server_keys[server].append(key)
            prefixed_to_orig_key[key] = orig_key

        return (server_keys, prefixed_to_orig_key)