Fix concurrent callback calls

[ddfisher]

Currently, gulp-batch will call the callback more than once concurrently if it gets an event while running the callback. This is because the module-level holdOn variable is not correctly set to true (the var holdOn = ... should just be holdOn = ...)!

Only removing the extraneous var would cause gulp-batch to completely ignore events while the callback is running, which is undesirable. This patch stores events that occur during the callback, and - if any occurred - waits for the timeout interval after the callback ends and flushes them.

[floatdrop]

Thanks! Published in 1.0.2

[UltCombo]

@floatdrop I've pulled gulp-batch's latest master to gulp-watch's dependencies, and gulp-watch's callback is only ever firing once now. Can you check this?

Make a simple gulp-watch task, then edit watched files twice or more. It logs the "was changed" message but does not fire the callback from the second time onwards.

[UltCombo]

@floatdrop given the current unit test that is failing, I'd guess asyncDone's callback is never fired.

[floatdrop]

@UltCombo I bet it because of you don't call callback or return Stream/Promise.

[UltCombo]

@floatdrop damn GitHub didn't show your comment automatically, and you're right. I had to dig it in the async-done docs:

Warning: Sync taks are not supported and your function will never complete if the one of the above strategies is not used to signal completion. However, thrown errors will be caught by the domain.

[ddfisher]

Thanks for the quick merge! :)