Feat; collector dequeue job

[Jamesbarford]

• Add a command for adding a collector to the database
• Add a command called DequeueJob (open to alternative names) that will take a job from the job_queue and print to standard out if a job exists.
• Exposed a few properties on the structs BenchmarkJob and CollectorConfig using pub fn <name_of_property>() methods
• More mapping functions between the collector / database enums
• Altered the job_queue table from having a collector_id to collector_name as the name column in the collector is unique, cannot be NULL and saves a lookup to get the id using the collector_name. I've retained the foreign key constraint.

[Kobzol]

Thanks! I will continue on top of this and try to join the dequeuing logic with actually running the benchmarks.