Allow throttling / delaying xhr progress updates

[kartik-v]

• New property uploadProgressDelay in microseconds - will default to 100 - this will control how frequent the xhr upload progress will be checked. If set to null or 0 - will do it every microsecond.
• New property maxAjaxThreads (will default to 5) that will allow to spawn only this limit of ajax requests in parallel.
• The above is complemented by resumableUploadOptions['maxThreads'] (defaults to 4) which is applicable for spawning number of ajax chunk requests for resumable uploads. The resumableUploadOptions['maxThreads'] property must be less than or equal to maxThreads - else it will be over-ridden by maxThreads global setting.

[kartik-v]

Resolved via this major upgrade to release v5.0.0 well ahead of time.

• Refer resumable uploads / chunk uploads documentation here
• Refer resumable uploads demos
• Refer enhancements and features closed in 5.0.0 release milestone