Task health status change notifications

The reusable health check program added in #22579 emits TaskStatus
messages when the task under supervision first becomes viable (when the
task passes its first health check).  It also emits a message when a
task changes state from healthy to unhealthy.

However, the scheduler should be notified for _every_ observed change in
health status.  It's easy to imagine cases where the scheduler wants to
wait a while before killing an unhealthy task, but still be notified of
status changes so that load balancers may be updated, etc.  This patch
therefore causes the scheduler to also be notified when an unhealthy
task becomes healthy again.

Review: https://reviews.apache.org/r/23966
Fixed a flaky slave recovery test.
Cleaned up the flag help documentation.

The --help on master and slave have a number of issues which are
resolved with this patch:
  1. Consistency - Path, location, where are all used to describe
     the path of a directory or file.  These are cleaned up.
  2. ZK path for a file must be file:/// not file://. I get that the
     last / is part of the path, however without an example it can be confusing to newbies
  3. --registration_backoff_factor help refers to a
     register_backoff_factor value which is incorrect.
  4. Attribute's notation really needs an example (which is added).

Review: https://reviews.apache.org/r/24057
MESOS-1616 Changed ReregisterCompletedFrameworks to use stout JSON framework

Addresses a TODO workitem in ReregisterCompletedFrameworks.  Manually parsed JSON for master and slave state now uses stout JSON utility classes.

Review: https://reviews.apache.org/r/23698