Improve feed endpoint customizability

[dankondr]

At the moment FeedEndpoint processes each feed item sequentially. I need to be able to customise how it runs tasks to parallel execution, thats why I moved items processing in dedicated method.

[dankondr]

Example of the new usage:

async def _process_items(...):
    if isinstance(items, AsyncIterable):
        await asyncio.gather(
            *(
                self._populate_feed(feed, item, request_is_secure)
                async for item in items
            )
        )
    else:
        await asyncio.gather(
            *(self._populate_feed(feed, item, request_is_secure) for item in items)
        )