[18.03] backport reaper fixes

[thaJeztah]

Backports for 18.03 of;

• [manager/orchestrator/task_reaper] Fix task reaper test to also set the desired state on tasks to prevent reconciliation races #2526 [manager/orchestrator/task_reaper] Fix task reaper test to also set the desired state on tasks to prevent reconciliation races
• [manager/orchestrator/taskreaper] Move task_reaper_test to orchestrator/taskreaper #2591 [manager/orchestrator/taskreaper] Move task_reaper_test to orchestrator/taskreaper
• [orchestrator/task reaper] Clean up tasks in dirty list for which the service has been deleted #2666 [orchestrator/task reaper] Clean up tasks in dirty list for which the service has been deleted
• [manager/orchestrator/reaper] Clean out the task reaper dirty set at the end of tick() #2675 [manager/orchestrator/reaper] Clean out the task reaper dirty set at the end of tick()
• Fix task reaper batching #2669 Fix task reaper batching

git cherry-pick -s -S -x a388cad309edddb9880899fe8927afbe4717a18e
git cherry-pick -s -S -x 8cfb337920a6658b302643f27074ca3d669176ec
git cherry-pick -s -S -x 592e8eddfa43ec5fbd6e34da5ad6890dfa9313fb
git cherry-pick -s -S -x 1a43a3b612d8c775db8a44c8399844e1f7e4aed2
git cherry-pick -s -S -x 5291c7a7b45773a4fe18720a54485ee2dde0af3d

Conflict when cherry-picking 8cfb337, likely because things were cherry-picked out of order;

On branch 18.03-backport_reaper_2
You are currently cherry-picking commit 8cfb3379.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   manager/orchestrator/taskreaper/task_reaper_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)

	deleted by them: manager/orchestrator/replicated/task_reaper_test.go

Resolved by git rm manager/orchestrator/replicated/task_reaper_test.go to mark resolution

To verify everything looked ok after merging, I compared the directories with master;

git diff master  manager/orchestrator/taskreaper/ 
git diff master  manager/orchestrator/

Which produced an empty diff, so the master/orchestrator directory is fully up to date with master

[thaJeztah]

This supersedes #2668

ping @anshulpundir @dperny @cyli PTAL if this LGTY

[codecov]

Codecov Report

Merging #2694 into bump_v18.03 will increase coverage by 0.33%.
The diff coverage is 82.6%.

@@               Coverage Diff               @@
##           bump_v18.03    #2694      +/-   ##
===============================================
+ Coverage         61.6%   61.94%   +0.33%     
===============================================
  Files              134      134              
  Lines            21795    21816      +21     
===============================================
+ Hits             13426    13513      +87     
+ Misses            6929     6858      -71     
- Partials          1440     1445       +5

[cyli]

LGTM, thanks for backporting @thaJeztah

[dperny]

LGTM, let's do it.