The semaphore mechanism should automatically detect when an other semaphore is outdated and so can/should be ignored

This semaphore mechanism was introduced in Sonar 3.4 to prevent two analysis on the same resource (project, view, ...) to be executed at the same time. But if an analysis is unexpectedly stopped for instance by killing the java process, the semaphore remains and it's then up to the Sonar administrator to use the property 'sonar.forceAnalysis' to ignore this past outdated semaphore.

We should prevent the Sonar administrator from having to take care about such situation by asking each alive semaphore to 'ping' the Sonar server. If a semaphore hasn't ping the Sonar server during the past X minutes we could then consider it as outdated.