Refs #2222 Implementing Visitor ID forced request, so that we can log, after the fact, a page/goal/action to the latest visit of a given visitor ID (useful for example to trigger goal conversions that were missed by the JS beacon...)
Had to add another index on the log_visit table unfortunately, to ensure reasonnably fast lookup on the idvisitor, even though this index will very rarely get used (never if you don't use the tracking API with setVisitorId()...)

git-svn-id: http://dev.piwik.org/svn/trunk@4369 59fd770c-687e-43c8-a1e3-f5a4ff64c105