Improve updateschema to create the indexes needed for an high performance xwiki

The aim of this issue is to help Wiki administrators automatically create MySQL indexes when their wiki starts being heavily used.

The use case is the following -> after a new wiki install, users start adding new documents and using the wiki. Once a wide number of documents are created, the wiki becomes slow and less responsive. Creating database indexes makes it snappy again. Thus automatically creating indexes for MySQL (which is the most widely used database for XWiki installs) would solve this issue in many cases.

The suggested interface would be an icon in the administration named database management. It would feature:

• List of databases that do not have their indexes created
• Automated index creation (if the DB is MySQL)

Ludovic was working on this with Raffaello to get the right script to improve an XWiki's indexes. They then need to improve the XWiki updateschema to verify the indexes are created and create them on the fly.