Count the distinct XWiki instances behind the Active Installs pings

The Active Installs script service could only count pings, and an instance sends many of them — one a day, plus one on every restart — so counting the instances themselves meant post-processing the pings by hand. Two new methods count instances directly: countDistinctInstalls returns the total, and countDistinctInstallsByExtension returns the breakdown by extension, computed in a single query rather than one query per extension. Both are experimental for now.

See Active Installs Script Service for the signatures, the precision limits and query examples.