[IMAPX] Do not hold queue_lock when calling imapx_server_ref_job()

Holding the queue_lock when calling imapx_server_ref_job() can cause
deadlock when the store has opened more than one connection and each
is searching for a duplicate job, both/all servers holding respective
queue lock prevents in a search of active jobs in them, both waiting
for a release of the queue_lock of the other server.