Simplify the lo/high partition offset computation

The computation of lo and high offsets used to be done in 2 steps:

• first get the watermak offsets (thus the absolute min and max offsets of the whole partition)
• then, as a "hook" in process(), retrieve the last committed offset for the partition and "push" these current offsets in the progress queue.

Instead, this simplifies a bit this process by quering the committed offsets while computing the hi/low offsets.

Also in this Diff:

• Use proper signature for JournalClientOffsetRanges.process()

• Explicitly close the temporary kafka consumer in get_offsets

  used to retrieve partitions and lo/hi offets.

  It could cause some dead-lock/long timeout kind of situation sometime (especially in the developper docker environment).

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Migrated from D6232 (view on Phabricator)