Validations for multiplicand min/max configuration

Ensure that "retry_delay_multiplicand_min" is less than or equal to
"retry_delay_multiplicand_max" (this applies to cases where both values
are specified or if only one or the other is specified and it conflicts
with the other multiplicand's default value)
Merge pull request #96 from jzaleski/jwz-validations-for-randomness-in-retry-delay

Validations for multiplicand min/max configuration