Add SugarUtils::File.flock_mutex

[acant]

Add this method which will use flock to ensure that the only one process can execute the given block at a time.

SugarUtils::File.flock_mutex(filename) do
  # code to run within the mutex
end

[acant]

There has been some discussion of the appropriate best practices for doing this:
https://www.ruby-forum.com/topic/77244

[acant]

Another stackover flow example of this pattern:
https://stackoverflow.com/questions/23748648/using-fileflock-as-ruby-global-lock-mutex-for-processes