If 'integers' default we generate sequential integers, it's assumed that all IDs are created
at one time, we are not adding to an existing set. Code that adds IDs should find
the largest existing ID and offset these by that value.
Using 'silicate.uid.type="uuid"' is the default. Using 'silicate.uid.type="integer"' is considered experimental.
By default UIDs are a mix of letters, LETTERS and digits of length getOption("silicate.uid.size")
which defaults to 6.
See ids
package for random_id
used if option 'silicate.uid.type="uuid"'.