Prevent command concurrency

[ptitFicus]

In some scenarios, commands should not be run concurrently.

There is several possible needs :

• Command should not be run concurrently with other commands of the same type on the same id
• Command should not be run concurrently with any other command on the same id

There is also several way to handle concurrent command :

• Keep second command waiting until events generated by the first command are consumed
• Reject second command