Convert dag.tasks list to dict to avoid slowdown

[jlowin]

After discussion with @mistercrunch, looks like #1213 introduced some slowdowns. I believe it's related to increased use of dag.get_task() which iterates over all tasks in dag.tasks. For example: https://github.com/airbnb/airflow/pull/1213/files#diff-a32a363fa616685db3bfefba947535b2R1767

The solution is to replace the list of tasks with a {task_id: task} dict. Adding a tasks property that returns a list of the dict's values means all other uses of dag.tasks should work.

[landscape-bot]

Code quality remained the same when pulling fd605d4 on jlowin:task-dict into 9689159 on airbnb:master.

[landscape-bot]

Code quality remained the same when pulling f7f2084 on jlowin:task-dict into 9689159 on airbnb:master.

[landscape-bot]

Code quality remained the same when pulling f7f2084 on jlowin:task-dict into 9689159 on airbnb:master.

[landscape-bot]

Repository health decreased by 0.04% when pulling e468e70 on jlowin:task-dict into 9689159 on airbnb:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• No problems were fixed.

[landscape-bot]

Repository health decreased by 0.04% when pulling 81bf781 on jlowin:task-dict into 9689159 on airbnb:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• No problems were fixed.

[mistercrunch]

Nice, I'm glad to find this, I'll just take it on from here

[r39132]

+1