Fix async pipeline control function defs

[HarshBalyan]

The asynchronous calls for pause/resume fail with

iex(1)> Flume.pause(:default_pipeline, async: true)
** (FunctionClauseError) no function clause matching in Flume.Pipeline.Event.Producer.pause/3

    The following arguments were given to Flume.Pipeline.Event.Producer.pause/3:

        # 1
        "default_pipeline"

        # 2
        true

        # 3
        5000

    Attempted function clauses (showing 1 out of 1):

        def pause(pipeline_name, false = _async, timeout)

    (flume) lib/flume/pipeline/event/producer.ex:29: Flume.Pipeline.Event.Producer.pause/3

The function definitions at

flume/lib/flume/pipeline/event/producer.ex

Line 25 in 1672003

def pause(pipeline_name, true = _async) do

and

flume/lib/flume/pipeline/event/producer.ex

Line 33 in 1672003

def resume(pipeline_name, true = _async) do

need to be fixed.