Don't check for active messages
Don't check for active messages

Instead of counting to check whether there are active messages, try to
fetch ids right away and use len on the result. This removes 1 query
from the claim process.

Fixes bug: #1207759

Change-Id: I579d09ed026d0889ded2463d4807154c8259109d

rackerlabs/marconi,openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar