Remove Transaction dependency on Dataset

[dhermes]

Instead imply the dataset ID from the mutations added throughout the transaction.

Needed to finish #477

[dhermes]

See #447 and #457 as well

[dhermes]

@tseaver I'm starting on this. LMK if you would rather do this than #485. IIUC you know more about the Transaction class than I do so I'm happy to yield.

[tseaver]

Go ahead -- I'm fine with either of them.