[Enhancement] tweaks for better error handling

[tom-pytel]

• Removed ContextManager.withSpanNoStop() to clean up interface since this was only meant to be used in plugins and so the plugins themselves should do this.
• Changed ContextManager.withSpan() and .withSpanAsync() to not insert span as first argument but rather just pass the args through to allow more generalized use, for example ContextManager.withSpanAsync(user_defined_span, unsupported_async_URL_requester, requester_arg1, ...).
• Reworked HttpPlugin:http.request() to better cover and handle error conditions, should be 100% covered now (before there was a range between span.async() and the callback where an unexpected error could cause the span to not be stopped).

[kezhenxu94]

Looks good