Tweak sys.alter_seq_restart helper

A few pieces:
- Make the underlying BasicDDLFunctions method transaction agnostic (like create, rename, etc already are)
- Allow first param to be NULL and second to be a name-string (e.g. from SERIAL_SEQUENCE)
- Change it from a procedure to a function

The facilitates a sophisticated single-statement usage (e.g. find sequence for identity, select max values and pass both to this helper) and is generally more convenient.