Add exists method on DOMStoreReadTransaction and DOMDataReadTransaction

Add exists method on DOMStoreReadTransaction and DOMDataReadTransaction

Introducing this API does not impact clients as it is mostly internal.

One reason for adding this API is because of ensureParentsByMerge used in a couple of places
which keeps checking if a parent node exists by reading a node to figure out if it exists. This is
fine for the InMemoryDataStore but it can be terrible in a distributed data store where the shard
which contains the data is remote. All sorts of overhead is associated with a remote read including
serialization which can actually be pretty expensive.

Change-Id: Ib5be5f6dc60be683d7a04c81dad08c56cd5681f4
Signed-off-by: Moiz Raja <moraja@cisco.com>