Better parameter parsing for strategy.

@enisoc @aaijazi 

Note I won't commit this CL until we also add the 2 other params: my delayAutoIncrement and Ammar's parameter to write to the masters. Then I'll add them to this CL and commit everything at once.
I'm not sure if you want to return err, or fmt.Errorf("cannot parse strategy: %v", err). Depends on which error you decide to return from NewSplitStrategy