[Utils] add new static function "retry"
[Utils -> retry] add docs for the return value
[Echo.Canvas] retry fetching a canvas in case of failure
Merge branch 'rc3.0.20' into neurostep/retry_canvas_fetching
[Utils -> retry] update logic to use exponential backoff algorithm, add tests;
[Echo.Canvas] rename "fetchRetries" config parameter to "maxConfigFetchingRetries";
update documentation
[Utils -> retry tests] use chain call instead of sequence of instructions
[Loader] increase "errorTimeout" parameter from 5sec to 10sec