Drop WindowPost tasks if deadline is past

[LexLuthr]

Right now we expect wdPost tasks related to deadline in past to not be executed. This was not seen practically in the case of a recent migration. We need to investigate the cause and design wdPost so we can drop the running task if deadline is already closed. Otherwise, 2 or more parallel running tasks cause extreme IO load on permanent storage. They can also lead to failure of current deadline wdPost due to slow read.