Fixed result timestamps when running multiple runs programmatically.

Earlier timestamps were generated based on global START_TIME that was
initialized when `robot` package was initialized. This resulted same
timestamps to be used on different runs.

Now internally used settings objects initilize their own
start_timestamp. When using `robot.run`, new setting objects are
always created for each run, and when using lower level
`TestSuite.run` its more common to give individual settings than whole
setting objects. If same setting objects needs to be reused when
timestamping outputs, start_timestamp needs to be reset between runs.

Fixes #1838.