[2.5] NXCM-5153: Core changes for upload resource

Smaller abstract upload resource changes and cleanup.

Changes:
* doc'd how upload actually works
* cleanup: variable and their scope cleanup, formatting cleanup
* minor logging cleanup
* and the "key change", introduction of new method called once all the coordinates of upload payload is available.

The key change is introduction of new method
`uploadGavParametersAvailable()` that is in this same
class implemented as nop. This method is called by
upload method only once, at the moment when
upload GAVP coordinates are available.