Expose number of queued messages in the API

Move the code to get the number of queued messages to
MassMessage::getQueuedCount(), and use the
'APIQuerySiteInfoStatisticsInfo' hook to add it.

Depends on I712748077a0ac2fb729bfe3b6616ac4981cb754e in core.

Change-Id: Ibfa77ec33fbf4a1c1d4807b37ef4b2414e4afcad
Merge "Expose number of queued messages in the API"