Batchspawner won't put the next job in a queue until the previous job is running

### Bug description
I've adapted Batchspawner to my own code (no Jupyterhub interface involved), using start() method to submit several jobs to a queue. However, I've noticed that Batchspawner won't put the next job in a queue until the previous job is running. This may cause a major waste of time when running several jobs on a cluster.

#### Expected behaviour
Batchspawner should queue all jobs immediately and leave them pending if they have to.

#### Actual behaviour
Batchspawner won't put the next job in a queue until the previous job is running.

### How to reproduce
Import Batchspawner as a module in your Python script and try submitting multiple jobs e.g. in a loop.

### Your personal set up

- OS: CentOS Linux release 7.6.1810 (Core)
- Version: Jupyterhub 1.1.0, Python 3.7.3
- Configuration: Slurm