Bug 8391 - Close producer in become-prefix-leader rpc implementation

MdsalLowLevelTestProvider's become-prefix-leader rpc implementation
creates CDSDataTreeProducer to try to move shard leadership. However,
the producer is not closed after leadership change request. This
prevents any subsequent invocations of become-prefix-leader rpc with
same prefix parameter to be successful. Subtree specified by the prefix
is attached to still opened producer and creation of any new producer
for this subtree fails. Close producer once we don't need it.

Change-Id: I3827e425082c35a43ec18dac1ef0f2dbd19b291f
Signed-off-by: Jakub Morvay <jmorvay@cisco.com>