ProgressMonitor: Refactor to use background alarms

ProgressMonitor: Refactor to use background alarms

Instead of polling the system clock on every update(1) method call,
use a scheduled executor to toggle a volatile once per second until
the task is done.  Check the volatile on each update(int), looking
to see if output should occur.

This limits progress output to either once per 1% complete, or once
per second.  To save time during update calls the timer isn't reset
during each 1% of output, which means we may see one unnecessary
output trigger if at least 1% completed during the one second of the
alarm time.

Change-Id: I8fdd7e31c37bef39a5d1b3da7105da0ef879eb84
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>