Only one call to fetch should return :commit

[whitfin]

References comments in #288 (comment). If you call the following 10 times, and my_func/1 takes a sufficient amount of time, "hello" will be printed 10 times.

with {:commit, _} = Cachex.fetch(cache, key, &my_func/1) do
  IO.puts("hello")
end

Realistically, only the first call to fetch should return the :commit tag and any queued listeners should return :ok so that it only occurs once with this pattern.

I'm going to label this an enhancement because it was never explicitly stated how this should work, but realistically I think this is probably a bug and should be corrected ASAP.