Upgrade Celluloid, remove defer usage

[mperham]

Use of defer was necessary when Celluloid message dispatch was done within fibers. This requires twice the number of threads. Remove it now that thread dispatch is the default.

[tarcieri]

Note that Celluloid::TaskThreads aren't actually the default. You'll want to do:

task_class TaskThread

...inside of processor to get this behavior. That said, TaskThreads are actually tested now :O

[mperham]

Good to know. Using an actual constant makes it hard to stub out Celluloid in testing. I'd prefer a symbol, e.g. dispatch_with :thread

Do I lose anything using threads rather than fibers? Will signals still work?

[tarcieri]

We can add a symbol as a shortcut.

Threads are a bit trickier to for us to manage and I think there may be some issues with shutdown that remain in 0.14 (specifically that the thread pool isn't correctly reaped.

In theory they're also more heavyweight, but it seems like the 4kB stack size is a far more pressing issue, and with defer you're using both anyway.