IndexTask: Use shared groupId when "appendToExisting" is on. (apache#…

…4582)

This allows the tasks to run concurrently. Additionally, rework
the partition-determining code in a couple ways:

- Use a task-id based sequenceName so concurrently running append
  tasks do not clobber each others' segments.
- Make the list of shardSpecs empty when rollup is non-guaranteed, and
  let allocators handle the creation of incremental shardSpecs.