Synchronous vs. Asynchronous methods

##Synchrony? A synchronous method completes its entire task before returning to the caller. An asynchronous method starts the task in the background and returns to the caller immediately.

##Some methods are more synchronous than others. In this project, there are two distinct meanings of "synchronous" method:

1. A method which blocks a Xojo thread. ("soft")
2. A method which blocks the application's one-and-only OS-level thread. ("hard")

A "hard" synchronous method will cause the entire application to block until it returns. In single-threaded console applications, this is just fine; in GUI applications it's an absolute disaster.

A "soft" synchronous method will block the Xojo thread that calls it, but will not block the entire application.

##Implications Most methods of EasyHandle, MultiHandle, ShareHandle, MultipartForm, and ListPtr are (or should be treated as) "hard" synchronous. For quick operations like setting an option or appending a form element this isn't a problem. For potentially very long operations like downloading a file, however, it is a major concern.

libcURL provides a solution via its curl_multi API. All the EasyHandles associated with a MultiHandle will perform a small part of their transfer and then return. This allows the "hard" synchronous nature of the method to be amortized over several invocations.

This means that a Xojo thread can invoke the method and then yield (if needed) to the next thread, or be rigged to invoke the method on the main event loop for asynchronous operation.

##tldr

All synchronous request methods of cURLClient are "soft" synchronous and will block the Xojo thread which invokes them; all asynchronous methods run on the main event loop.