Refs #1938 : Recurring tasks - scheduler backend

A recurring task will be stocked as a draft into the msc database.
To identify our draft, we can use an unused field commands.state
with a default value 'active'.

This commit includes :

- adding a new element to enumerated field commands.state
- adding a new filter to all queries to select only 'active' commands
- state is taken in acccount when a command is created

Creating of a draft is now possible via xmlrpc including element
{state : 'draft'} in the dictionnary $params.