Issue #121: Added transaction indexes.

Closes #121 
Closes #122 

@rcrichton if you could please review?

Added indexes for `request.timestamp`, `channelID` and `status`. Anything I may be missing?

I thought about doing `response.status` as well, but currently the only queries that uses it (error alerts) filter by timestamp first anyway, so I'm not sure how useful that index would be.