Handle concurrent state updates

[kettanaito]

What

I propose to provide a consistent reliable way of handling concurrent state updates.

Why

At the moment some concurrent updates result into obsolete fields. That causes various unexpected behaviors.

How

1. Accumulate fieldChange during a certain period of time and handle all the accumulated payload at once. Con: That would mean an idle accumulation period, and "synced" patch state update.
2. Get at-point fields reference on form.handleChange. In other words, maybe add a second parameter to form.updateFieldsWith(fieldProps, fields) to have explicit fields passed.

This change is not going to affect the public API and thus can be released as a minor version.

[kettanaito]

This issue results into numerous bugs, mainly connected to keeping controlled inputs in a form. This needs to be resolved. I will try to think of a possible solutions and pick the best at the moment.

[kettanaito]

I've summed it up in one of the pull requests, but I am going to repeat this here too:

The concurrency issue can be eliminated using a simple bufferTime during the event observation. That, however, bears no effect over the typing-based value updates of controlled fields. The issue lies in dispatching an event in Field.componentWillReceiveProps, which is a no-op. This may be a good reason to refine Form-Field communication pattern.