result.sampling_time does not persist on restarting jobs

https://git.ligo.org/lscsoft/bilby/blob/master/bilby/core/result.py#L135

sampling_time: float
    The time taken to complete the sampling

Sampling time (in seconds) is the machine time duration between the time when

• run_sampler is called
• sampler returns its results

When a job is terminated and restarted, the timing of the duration is restarted. It'd be nice if the duration could persist between jobs.