Extract Celluloid::IO::Stream::Latch into Celluloid itself

[tarcieri]

This class is effectively a "task aware mutex" and is available here:

celluloid-io/lib/celluloid/io/stream.rb

Line 369 in fdd6644

class Latch

Perhaps it should be called Celluloid::Mutex instead? Celluloid::Lock? Celluloid::Semaphore? (it's close to being a semaphore)

[digitalextremist]

This will be done in 0.17.5 at celluloid/celluloid#341