rangefeed: clean up task code

We have some code in `kvserer/rangefeed/task.go` which should be cleaned up. It implementes a `runnable` interface that isn't really used anywhere, and it's tightly coupled with the processor ([soon](https://github.com/cockroachdb/cockroach/pull/107377) via a `processorTaskHelper` interface). I think the task idea may possibly be a historical vestige of some former structure. We should restructure these more like standalone helper functions, where the processor itself takes action based on the outcome of the functions.

See https://github.com/cockroachdb/cockroach/pull/107377#pullrequestreview-1565075260.

Jira issue: CRDB-30596