Change length to minLength and add maxLength to sequential identifier configuration

When a sequential identifier's seed reaches the maximum length, an attempt to generate the next identifier causes idgen to throw an exception.

Changing length to minLength and adding maxLength will give us the flexibility to allow an identifier to grow beyond the expected limitations. Both of these properties should have a default of null or zero. For minLength, this means there will be no padding. For maxLength, this means it can grow indefinitely.