Add task rate limiting

[Olivier Sallou]

Add possiblity to limit max number of task pending for a user or globally

Above limit, tasks are rejected.

This will limit the number of pending tasks to avoid overload of scheduler.

User should send batch tasks (10 per 10 for example).

[Olivier Sallou]

add config variable rate_limit (per user) and rate_limit_all

[Olivier Sallou]

add rate limit, closes #25

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