100% CPU with multiple subscriptions

Calling pn_messenger_subscribe() twice (which seems to be perfectly valid) can result in proton hanging at 100% cpu, when one of the two sources is unreachable.

To reproduce, start the attached test case against a pristine ActiveMQ copy (./bin/activemq console). Proton will go nuts when trying to connect to the unavailable amqp://localhost:15672/ganymede instance.