Make InMemoryDOMDataStore.ready() unsynchronized

Tracing datastore operations has revealed some contention on the
datastore, specifically when users call WriteTransaction.ready(). It
seems this method does not need synchronization at all, as it is not
directly changing any state.

Change-Id: I635f6a925694e4b68e629978fbd535fedb3a6cd6
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Make InMemoryDOMDataStore.ready() unsynchronized"