Normalize courier calls to simplify :commit handling

[whitfin]

This fixes #290.

I'm going to label this as a bug because I think it was unintended, and thus has now been fixed. Currently if you execute many fetch/4 calls at the same time, it becomes impossible to know which of them actually wrote a value to the cache:

iex(1)> Cachex.start(:test)
{:ok, #PID<0.528.0>}
iex(2)> my_func = fn _key -> :timer.sleep(2000) && "value" end
#Function<44.65746770/1 in :erl_eval.expr/5>
iex(3)> for _ <- 1..10 do
...(3)>   spawn(fn -> IO.inspect(Cachex.fetch(:test, "key", my_func)) end)
...(3)> end
[#PID<0.539.0>, #PID<0.540.0>, #PID<0.541.0>, #PID<0.542.0>, #PID<0.543.0>,
 #PID<0.544.0>, #PID<0.545.0>, #PID<0.547.0>, #PID<0.548.0>, #PID<0.549.0>]
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}
{:commit, "value"}

This makes chaining calls on (such as then providing an expiration for your record) will run for every call instead of just the one which triggered the write. Ideally, this would run once in the process which was first to initiate the fetch. This PR fixes this to be the case, thus making the recommended expire pattern work correctly.

iex(1)> Cachex.start(:test)
{:ok, #PID<0.528.0>}
iex(2)> my_func = fn _key -> :timer.sleep(2000) && "value" end
#Function<44.65746770/1 in :erl_eval.expr/5>
iex(3)> for _ <- 1..10 do
...(3)>   spawn(fn -> IO.inspect(Cachex.fetch(:test, "key", my_func)) end)
...(3)> end
[#PID<0.539.0>, #PID<0.540.0>, #PID<0.542.0>, #PID<0.543.0>, #PID<0.544.0>,
 #PID<0.545.0>, #PID<0.546.0>, #PID<0.547.0>, #PID<0.548.0>, #PID<0.549.0>]
{:commit, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}
{:ok, "value"}

With this in place, chained calls based on :commit will only fire in a single process instead of all of them. This is far better ergonomically, and also in terms of performance as we're not wasting many cache calls unnecessarily.