Handle Updates to Matchgrid Sequence Settings

If a Matchgrid is configured to assign Sequence based Reference Identifiers, and the Initial Value is updated, nothing happens since the value is only used when the sequence is created. This is probably for the best, since there is no uniqueness constraint on the reference id column (by necessity), and no in-use check, so it would be possible to assign an existing Reference ID.

Minimally, the interface should prevent changing the initial value. As a general solution, the identifier generation routines should check that the newly generated identifier is not already in use. This could apply to the UUID service as well, though the likelihood of a conflict there is pretty trivial.