As found in see NETVIRT-883 but certainly not causing CONTROLLER-1756 but should probably still be fixed, unless it's proven false positive.. in which case, if confirmed after analysis, this issue should be closed RESOLVED INVALID with an "explanation" why it may be perfectly logical to see non closed ReadOnlyTransaction created by BridgeConfigReconciliationTask:

trace:transactions
TracingBroker found some not yet (or never..) closed transactions!

DataBroker : newReadOnlyTransaction()
28x transactions opened here, which are not closed:
org.opendaylight.ovsdb.southbound.ovsdb.transact.BridgeOperationalState.<init>(BridgeOperationalState.java:65)
org.opendaylight.ovsdb.southbound.OvsdbDataTreeChangeListener.updateData(OvsdbDataTreeChangeListener.java:214)
org.opendaylight.ovsdb.southbound.OvsdbDataTreeChangeListener.onDataTreeChanged(OvsdbDataTreeChangeListener.java:100)