Extend cluster-wide lock to Workflow model import

In cluster mode, the worfklows are initialized non-concurrently in a cluster-wide critical section. When a cluster node attempts to initialize its workflows and another node is already doing the same thing, it will wait for 1 min for the cluster-wide lock to be released and do its own initialization. If this timeout expires, then initialization fails with an exception. The following nuxeo.conf properties can be used to change this timeout: org.nuxeo.workflow.cluster.start.duration=1m In case where there's a startup crash while a lock is held, it may be necessary to manually cleanup the key/value store of its lock. The key corresponding to the lock is visible when using Redis with KEYS nuxeo:cluster:* , in this case nuxeo:cluster:start-workflows-default for the initialization of the workflows of the default repository. For a MongoDB key/value store, the keys are stored in the collection  kv.cluster