life-cycle: resource status change listener

[myfreeer]

This would enable invokers to save the status, to file or database. Another issue should be fired to enable resources being loaded to a downloader.

Resource status should contain:

• createResource
• processBeforeDownload
• download
• processAfterDownload
• saveToDisk
• error
• dispose

The dispose status invoked if a resource passed all the life cycles, or interrupted by a void return of a life-cycle function, or a shouldBeDiscardedFromDownload prop.

A status change listener should be invoked concurrently, ignoring thrown errors. Invokers should decide whether to return a promise or not. A status change listener should conider the resource or any else parameter passed to it readonly, and the behavior of changing it considered UB.

We might use add EventEmitter to pipeline or downloader.