Add a multi-subscription status check

[fdaugan]

Add a multi-subscription status checks contract. Currently, there is only a single subscription status check contract. However, we often need to check several subscriptions related to a same tool, so the same set of parameters.
For some tools, it is possible to group these checks into a single query to the tool, or at least would be faster than single checks.
The SubscriptionResource class should be able to :

• accept a collection of subscription check requests having nothing in common
• split a collection of check requests and group them into sub-set of requests for tools supporting this feature and sharing the same node.

This feature brings a huge performance gain.