Change Promise callback invocation order

Callbacks can't be executed always-sync and in the same order as registered in the edge case where another when handler is registered within a when handler. Once a promise is resolved, all new when handlers must be executed immediately now.

Author: kelunik

README.md

@@ -72,7 +72,7 @@ Any implementation MUST at least provide these two parameters. The implementatio
 
 > **NOTE:** The signature doesn't specify a type for `$error`. This is due to the new `Throwable` interface introduced in PHP 7. As this specification is PHP 5 compatible, we can use neither `Throwable` nor `Exception`.
 
-All registered callbacks MUST be executed in the order they were registered. If one of the callbacks throws an `Exception` or `Throwable`, it MUST be forwarded to `Async\Interop\Promise\ErrorHandler::notify`. The `Promise` implementation MUST then continue to call the remaining callbacks with the original parameters.
+All callbacks registered before the resolution MUST be executed in the order they were registered. Callbacks registered after the resolution MUST be executed immediately. If one of the callbacks throws an `Exception` or `Throwable`, it MUST be forwarded to `Async\Interop\Promise\ErrorHandler::notify`. The `Promise` implementation MUST then continue to call the remaining callbacks with the original parameters.
 
 If a `Promise` is resolved with another `Promise`, the `Promise` MUST keep in pending state until the passed `Promise` is resolved. Thus, the value of a `Promise` can never be a `Promise`.