More transaction metadata

[lukechampine]

/transactions/:id currently reports the inflow, outflow, and feePerByte in addition to the raw transaction. Those things are nice, but having the block height, block id, and timestamp would be far more useful.

This shouldn't be too tough to implement. wallet.ProcessedConsensusChange turns a consensus change into a flat []types.Transaction, so that type would just need to be changed to include the block-specific metadata. Then the implementers of wallet.ChainStore would need to store this extra information.

The tricky part is handling future changes. It's not hard to imagine that we'll want to add additional metadata later. When we do, will all the existing stores need to run some upgrade code? Will some of their transactions end up with missing metadata? Not sure how best to handle this.

[lukechampine]

Implemented in lukechampine/us@f35d97a and 6625a86