Netflix / genie / 1140

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travis-ci

Add simple user job limiting mechanism (#517)

Motivation:
Introduce a simple mechanism that allows administrators to set a upper limit to the number of jobs users are allowed to concurrently run. The limit, if enabled, is applied to all users.
This mechanism is designed as a first line of defense against a user issuing an avalanche of job submissions (either maliciously (i.e. to cause denial of service), or unintentionally (i.e. client-side bug).

Change:

 * Implement database query that returns the number of active jobs for a given user (in JPAJobSearchService)
 * Introduce new properties to toggle and configure the per-user active jobs limit
 * Introduce new GenieException to model a user limit exceeded
 * Reject job submissions (in JobCoordinationService) if a user exceeded the limit

Note:
It is still possible for a user to issue more jobs than the limit.
For example, by issuing two jobs on two genie instances at exactly the same time, both of them will check the limit and let a job through, even if one one of them should have.

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