implement structured logging in go-ethereum

[sveneh]

Implement the structured logging spec from @heikoheiko defined at https://github.com/ethereum/system-testing/wiki/Log-Events

This is a requirement for ethereum/system-testing#30

Will also be done for cpp, tracked here: ethereum/aleth#705

[obscuren]

@tgerring Could you look in to this :)

[tgerring]

Will do!

[fjl]

Sven is telling me that this is really urgent. @tgerring did you have a chance to look at this yet?

[tgerring]

@obscuren did you have a chance to document the logging changes you had in mind?

[obscuren]

Yes but it will involve a full re-write of the logging system. Putting it up tomorrow

[obscuren]

Ignore my earlier statement regarding reimplementing the log system. Instead we'll add additional logging.

We'll use our current event system for passing LogEvents to the ObjectLogger object. The ObjectLogger should be started as a service when requested from the CLI and watch for LogEvents. @tgerring I'll leave the implementation of how the ObjectLogger interprets events to you.

Example

// Event type
type LogEvent struct {
    // Additional fields?
    Object interface{} // json.Marshal this object
}

// Post object
eventMux.Post(LogEvent{myObject})

// Handle object
switch ev.(type) {
    case LogEvent:
        json.MarshalIndent(ev.Object)
}