Fix halo-enabled option

Summary:
- This option was broken (or shall we say....not fully implemented? :) ),
  it needs to set the halo-latency to FLT_MAX to
  gaurentee no bricks will be marked down, and all those presently
  marked down @ runtime shall be marked back up upon the transition from
  true to false
- Also fixes tests/bugs/fb2518260.t

Test Plan: Prove tests (paste coming)

Reviewers: cjh, dph, mmckeen

Reviewed By: mmckeen

Differential Revision: https://phabricator.fb.com/D1435264

Conflicts:
	xlators/cluster/afr/src/afr-common.c

Change-Id: I81209d6f2cc9ea7e562eedf44bf3efbe87e01bf7
Signed-off-by: Kevin Vigor <kvigor>
Reviewed-on: http://review.gluster.org/16227
CentOS-regression: Gluster Build System <jenkins.org>
NetBSD-regression: NetBSD Build System <jenkins.org>
Reviewed-by: Shreyas Siravara <sshreyas>
Smoke: Gluster Build System <jenkins.org>