Create CPU Quota framework

[prioux]

We have already a nice disk quota framework implemented as ActiveRecords. The system works through callbacks during the userfiles save() method, and can limit a specific user (or all users) to a maximum amount of bytes or files on a DataProvider.

I think we should create a parallel, very similar framework for limiting the amount of CPU a user request. When the quota is exceeded on a Bourreau, the user would be prevented from creating new CbrainTasks on it.

The check would be performed by a sum() of the values of the records in the ResourceUsage table, which are all persistent even when users delete their tasks.

We have two approaches for this feature:

1. a distinct table (DiskQuotas use the table 'disk_quotas' and CpuQuotas would use 'cpu_quotas')

• this would imply recreating a full new interface to query and set the quotas

2. we rename 'disk_quotas' to 'quotas' and add a 'type' attribute, turning the table into a multiple inheritance table

• this would allow use to use the current interface and adjust it to support the two types of quotas
• we'd need to add columns "remote_resource_id" and "max_cpu"
• the type of quota determines which attributes are meaningful

[prioux]

Unlike for disk quota, we also need a time range. Weekly quota? Monthly? Total max limit too? These would all require separate attributes.

[MontrealSergiy]

I would mimic the Slurm or mobile operators - the more you use, the slower you speed e.g. adding artificial delays after reaching the quota, proportional to overuse and/or number of scheduled and running tasks