'Init.
Args:
todo_tasklet: the tasklet that actually fires RPC and waits on a MultiRPC.
It should take a list of (future, arg) pairs and an "options" as
arguments. "options" are rpc options.
limit: max number of items to batch for each distinct value of "options".'

def __init__(self, todo_tasklet, limit):

    self._todo_tasklet = todo_tasklet
    self._limit = limit
    self._queues = {}
    self._running = []
    self._cache = {}
