Minion Realtime To Offline Segment Task runs slow as there is no parallelism per task

1 minion host running 1 real-time to offline task only allows 1 subtask to run at a time. Since there are no parallelism, minion cannot scale if the realtime table becomes too huge.

Note: Mapper phase is usually the bottleneck as observed from my logs.

Start by tracing the Minion Realtime To Offline Segment Task and its Mapper phase, using the reported one-subtask-at-a-time behavior as the entry point. Determine where task parallelism is constrained and define done as allowing useful parallel subtasks so a large realtime table can scale better.

Written by the indexing model from the issue text.