Fixes soft assertion in NonblockingBatch: Closes #4393
Fixes soft assertion in NonblockingBatch: Closes #4393

Problem was due to an internal private class of NonblockingBatch holding a weak ref
to its owner.  Now holds a strong ref when scheduled and drops it when the operation
is completed, to prevent a reference cycle.

robol/geary,tshikaboom/geary,kpiwko/geary,didrocks/geary,mjog/geary,githubnemo/geary,jimbonator/geary,geekvigilante/geary,glebihan/geary,rschroll/geary,glebihan/geary,emersion/geary,glebihan/geary,kyoushuu/geary,aatishnn/geary,rschroll/geary,mpdeimos/geary,emersion/geary,orlandosanchez/geary,aatishnn/geary,robol/geary,tshikaboom/geary,tshikaboom/geary,githubnemo/geary,robol/geary,martinra/geary,MohamedIM/geary-unofficial,AtticHacker/geary,emersion/geary,kyoushuu/geary,dominickm/Geary,jimbonator/geary,martinra/geary,mjog/geary,orlandosanchez/geary,orlandosanchez/geary,AtticHacker/geary,martinra/geary,jimbonator/geary,abn/geary,aatishnn/geary,githubnemo/geary,AtticHacker/geary,mjog/geary,dominickm/Geary