MINOR: activity: report the average loop time in "show activity"

MINOR: activity: report the average loop time in "show activity"

Since we know the time it takes to process everything between two poll()
calls, we can use this as the max latency measurement any task will
experience and average it.

This code does this, and reports in "show activity" the average of this
loop time over the last 1024 poll() loops, for each thread. It will vary
quickly at high loads and slowly under low to moderate loads, depending
on the rate at which poll() is called. The latency a task experiences
is expected to be half of this on average.