A transaction handle uniquely identifies a transaction on the storage backend.
All modifications to the storage backend must occur within the context of a single
transaction. Such a transaction is identified to the Titan middleware by a StoreTransaction.
Graph transactions rely on the existence of a storage backend transaction.
Note, that a StoreTransaction by itself does not provide any isolation or consistency guarantees (e.g. ACID).
Graph Transactions can only extend such guarantees if they are supported by the respective storage backend.