node-bitcoin-exit: RPC API to get transaction information

[killerstorm]

We need to be able to get data for any transaction. Currently node-bitcoin-exit provides no such facility, so we get it by proxying requests to bitcoin-tx-spent-db.

This RPC API should be available via socket.io just like other APIs provided by node-bitcoin-exit.

Bonus feature: ability to query many transactions at once.

Bounty: 0.5 BTC

[katuma]

See katuma/node-bitcoin-exit@d8f77b6 and katuma/node-bitcoin-exit@fd67064

Still needs some testing.
As a consequence, there is no longer dependency on bitcoin-tx-spent-db, query.js changes are minimal as possible to ease porting of future changes in spent-db.