Remove transaction ID maps

[chris124567]

Finish #208 and remove the remaining maps that we used to map Sia IDs to internal database IDs.

christopher/clean-up-indexing-more:

----- latest commit (8946...) -----
BenchmarkApplyRevert/apply-4                 276       4194136 ns/op
BenchmarkApplyRevert/revert-4                 34      33399087 ns/op

master:

BenchmarkApplyRevert/apply-4                 248       4679242 ns/op
BenchmarkApplyRevert/revert-4                 33      33931566 ns/op

[n8mgr]

Lots of duplicated work here. Calling txn.ID() and getTransactionID() in every function is unnecessary.

Most of these methods only use the transaction to calculate the ID and access the slice. Instead of passing the transaction, you can pass exactly what you need:

func addMinerFees(tx *txn, txnID int64, []types.Currency)
func addSiacoinInputs(tx *txn, txnID int64, []types.SiacoinInput)

For the ones that need the transaction data to derive an ID, you should still pass the txn, but I'd recommend passing in the db ID as an additional arg instead of grabbing it. Goes for the V2 code as well.