Background queue for task verification

[KrzysztofMadejski]

This is an optimization for verifying tasks. Currently tasks are being crossverified in the request-cycle on new entry being sent that passes MIN_ENTRIES_TO_CROSSCHECK limit.

Acceptance criteria:

• when given task receives a set number of runs the task is taken into verification queue (and maybe taken out of to-do queue, see q1 below)
• process runs and verifies each task Custom rules for auto verification #73
  • if verified the task should be taken out of to-do queue
  • if not verified task should be flagged to manual verification Manual verification #5

Questions:

1. we don't mind more entries than minimum number, should we allow more entries and take out tasks only when verified? ANSWER: Yes, we do allow more entries. Up to MIN_ENTRIES_TO_MARK_DIRTY setting. But even when the task is 'dirty' on new entries that started in the meantime we will perform cross-check.

See #84 for background discussion on that.