Exceptions when multiple concurrent restconf clients are trying to add flows to the controller.

See OPNFLWJAVA-29 (https://bugs.opendaylight.org/show_bug.cgi?id=1373): in addition to the openflowjava null pointer exceptions, there are md-sal null pointer exceptions.

for example:
2014-07-15 23:02:08.671 PDT [pool-8-thread-1] ERROR o.o.c.m.s.b.i.ForwardedBackwardsCompatibleDataBroker - Transaction DOM-325 failed to complete
org.opendaylight.controller.md.sal.common.api.data.OptimisticLockFailedException: Optimistic lock failed.
at org.opendaylight.controller.md.sal.dom.store.impl.InMemoryDOMDataStore$ThreePhaseCommitImpl$1.call(InMemoryDOMDataStore.java:321) ~[na:na]
at org.opendaylight.controller.md.sal.dom.store.impl.InMemoryDOMDataStore$ThreePhaseCommitImpl$1.call(InMemoryDOMDataStore.java:311) ~[na:na]

or

java.lang.IllegalStateException: null
at com.google.common.base.Preconditions.checkState(Preconditions.java:133) ~[bundlefile:na]
at org.opendaylight.controller.sal.dom.broker.impl.SchemaAwareRpcBroker.invokeRpc(SchemaAwareRpcBroker.java:261) ~[na:na]
at org.opendaylight.controller.sal.dom.broker.impl.SchemaAwareRpcBroker$RoutedRpcSelector.invokeRpc(SchemaAwareRpcBroker.java:360) ~[na:na]
at org.opendaylight.controller.sal.dom.broker.impl.SchemaAwareRpcBroker.invokeRpc(SchemaAwareRpcBroker.java:186) ~[na:na]
at org.opendaylight.controller.sal.dom.broker.osgi.RpcProvisionRegistryProxy.invokeRpc(RpcProvisionRegistryProxy.java:67) ~[na:na]
at org.opendaylight.controller.sal.binding.impl.connect.dom.RpcInvocationStrategy.forwardToDomBroker(RpcInvocationStrategy.java:121) ~[na:na]
at org.opendaylight.controller.sal.binding.impl.connect.dom.BindingIndependentConnector$DomToBindingRpcForwarder.invoke(BindingIndependentConnector.java:718) ~[na:na]

See OPNFLWJAVA-29 for the logs and how to reproduce.