Add configurable base seed

[jjk-g]

What would you like to be added:

#340 introduced a per-worker seed for random.

The functionality should be extended so that

1. A single base seed can be passed in via the config. Workers should derive from this base seed. Default is current time.

Why is this needed:
This enables exact reproduction of random runs.

[changminbark]

I can take this issue.

[changminbark]

/assign