InMemoryDOMDataStore$DOMStoreTransactionChainImpl isnt happy if there are mulitple transactions queued in quick succession.

And it throws
java.lang.IllegalStateException: Previous transaction in chain must be ready.
at com.google.common.base.Preconditions.checkState(Preconditions.java:149) ~[na:na]
at org.opendaylight.controller.md.sal.dom.store.impl.InMemoryDOMDataStore$DOMStoreTransactionChainImpl.newReadWriteTransaction

when we reuse the txchain. Fixing it temporarily. Better solution is to get away from this and use DataChangeListener.

Change-Id: Id20abdefc21574dce9ff078f4ce96aa91c390edb
Signed-off-by: Madhu Venugopal <mavenugo@gmail.com>