Refactor the PusherConsumer interactions

- Introduce a new `BatchingQueue` to reveal the intentions of a queue per shard.
- Removed the need to call `Close` as we pushed data into TSDB from the main loop. This was really confusing given we were using close as a semantic to "no more items are coming" and ensure any incomplete batches were done.
- Renamed and moved the `noopPusherCloser` which is more of an alternative way to push data without any sort of concurrency.
- Inlined a few methods to get rid of certain level of indirection that made the code harder to understand.