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.