fixup comment
use stringstream to get seeds from configs, not lexical cast

Also, get seed values for configs from the get_random_seed_str
function.

We don't otherwise mess with the types of synced_context etc.
The basic types of those objects, seed_generator etc. is the
same except that int -> uint32_t.

Fixes up commit cdefb9c98e8517c5c2b532970fdaf22d8c1b73de