stop reseeding each call to solve

[isaacsas]

By default JumpProcesses reseeds with a rand(UInt64) on every call to solve. But there is no real reason to do this for a sequence of solve calls running on the same thread, and it leads to undesirable effects like non-reproduciblity of results (because it doesn't respect the user's rng choice).

It would be breaking, but I think we should drop reseeding if no seed is passed. If this has an impact on EnsembleProblems, then they should be updated to properly handle passing new unique seeds to spawned solvers when needed instead of trying to put this decision at the level of solve.