rpk topic consume: expand the offset flag

We now:

- can consume through the end and quit
- can consume based on time
- have many options for representing time ranges and durations

Previously, consuming to an end offset left the client running in the
paranoid case that a new partition could be discovered and consumed to
that end offset. Now, we take a snapshot of the end offsets when rpk
starts, consume to those, and quit.

Closes #4073.