Enable batch notifications for SimplePush server

Currently we are following the specs with regards to handling notifications. This task purposes to enhance this to not only allow clients to PUT a single update but to support list of pushEndpoints to be notified.
For example, a client could PUT to https://hostname:port/update with a body containing a JSON object similar to this:

{"version": 3, "pushEndpoints": ["pushEndpoint1", "pushEndpoint2"]}

All the pushEndpoints will be sent the version that is specified. The entries in the pushEndpoints array should be the encrypted endpoint path, for example, if the pushEndpoint returned from a channel registrations is:

https://timeout-dbevenius.rhcloud.com:8443/update/5SrxPo9fEnfmvOLvaIs9hMIEPYDTTJiHz2ox1dNTxw9qMeF-iRaJOJ1Y1SkHB1JjJT8zM5Kmt5iNKmHX9J-FIA

The entry for this in the pushEndpoints array would be:

5SrxPo9fEnfmvOLvaIs9hMIEPYDTTJiHz2ox1dNTxw9qMeF-iRaJOJ1Y1SkHB1JjJT8zM5Kmt5iNKmHX9J-FIA

The benefit of doing this is that to support a form of broadcast from a backend server, like our UnifiedPush Server. Currently, to implement this multiple single http calls have to be made which is time consuming. This way only a single http request is required.

Note that this would be a suggestion for the SimplePush protocol and will need to be accepted by Mozilla