Move event attribute classes to a separate package

The Attribute classes are moved from:

  com.google.gerrit.server.events

to the newly created package:

  com.google.gerrit.server.data

The `QueryStats` class is renamed to `QueryStatsAttribute` to be
consistent with the other classes in the package.

This is only a code reorganisation.  No functionality is changed.

Change-Id: I5c125e8d45d8f8e6943021cd90ddba1c364a103e
25 files changed