IMDS: trim down commit overhead

After refactoring things out it is quite obvious that we can skip adding
a listener to the commit cohort when committing a chained transaction.

Extract the inner class from InMemoryDOMDataStore so it can be reused
and subclass it in ChainedTransactionCommitImpl. That makes it also
obvious our commit cannot fail.

Change-Id: I71d9fa60cf63d826ba066c381d8db91af7f8f31e
Signed-off-by: Robert Varga <rovarga@cisco.com>