MINOR: activity: clean up the show profiling io_handler a little bit

MINOR: activity: clean up the show profiling io_handler a little bit

Let's rearrange it to make it more configurable and allow to iterate
over multiple parts (header, tasks, memory etc), to restart from a
given line number (previously it didn't work, though fortunately it
didn't happen), and to support dumping only certain parts and a given
number of lines. A few entries from ctx.cli are now used to store a
restart point and the current step.