Feature: Distinguish between scheduling and assignment of jobs

[cburgmer]

Go.cd (and other's maybe as well) first schedule a job after an external event (e.g. source change), and when a build node gets available assigns a job.

Including the delay between both in the job runtime in statistics is misleading, as an increased timing there indicates that build system might be overloaded, rather than the actual step taking more time. However monitoring the delay is important when planning to reduce cycle time.