mod_rest: Declare presence priority an integer in -128..127 per RFC6121 util.datamapper doesn't actually understand min/max, but it's informative and the schema could be used to validate the input at some point.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```