Launcher job limit

This MR implements a job-limit feature.

It uses two new options, timer_delay and job_limit:

• before every job submission the number of alive jobs will be compared to job_limit. If greater, the submission will be postponed otherwise it will be processed.
• the timer _interval (i.e. time interval triggering timeout transition) as well as the job_update_interval (i.e. minimal delay between two job status updates) are initialized with timer_delay.