After #509 is in, We need to decouple Transaction from Connection and allow Transaction to inherit from Batch.

Things to be done

• Change Transaction to inherit from Batch (Derive 'Transaction' from 'Batch'. #516)
• Track "active" batches / transactions in a thread-local stack (Push batch / transaction onto a thread-local stack inside 'with'. #518)
• Replace Key.get() with top-level datastore.get() (Rename get_entities() to get(). #520, Allow get() to take a single key (in addition to a list) #521, Remove Key.get() and Entity.reload() #522)
• Replace Entity.save() with top-level datastore.put() (#514: add datastore.put API, remove `Entity.save' #548)
• Replace Key.delete() witg top-level datastore.delete() (Remove Key.get() and Entity.reload() #522)
• Remove non-RPC methods save_entity() and delete_entities() from Connection (#514: rip out Connection cruft #550)
• Remove Connection.mutation(). (#514: rip out Connection cruft #550)
• Remove Connection.transaction() (#514: rip out Connection cruft #550)

Also relevant: #447 (Transaction and Batch should not have a dataset_id until they have objects to work with)

For reference, RPC methods are:

• Lookup
• RunQuery
• BeginTransaction
• Commit
• Rollback
• AllocateIds