Feature - Drop old tasks with max_count or timedelta

[denisolyanyuk]

It would be useful to have the ability to drop tasks in flower: in my case there is a task that runs pretty frequent (for example 2 every mins) and it's important to see last 10 runs and would be better to drop all not important tasks.

This solution is similar to this PR #1188 but I think my approach is better.
You need to add one list in flowerconfig.py and that's all:

limit_tasks_by_type=[ {'type': 'test_task', "max_count": 10, "timedelta": timedelta(minutes=10)}, ]

It will keep 10 newest test_task and drop everything else and drop any test_task older that 10 minutes

[max-wittig]

@mher Could you take a look at this one? That seems to be a missing feature in Flower.

[premchalmeti]

This is required. @mher any plans on merging this?