StatsUtil#computeUniqueID use an hardcoded value for the maximum size of a unique id

Instead of using an hardcoded value it should use the store API to ask what is the actual limit.

See https://github.com/xwiki/xwiki-platform/blob/1e98576b9e37399aed66221c3e222b8d07187390/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/stats/impl/StatsUtil.java#L550