Add Logs API to monitor

[htdvisser]

This PR adds a rpc Logs(stream LogMessage) to the monitor API.

• Only changed file is monitor/monitor.proto, the rest is generated code.
• Documented guidelines (levels, messages, fields) correspond to our internal TTI guidelines
• Fields are attached as a "simple" google.protobuf.Struct which basically implements a map[string]Value where Value can be one of nil, float64, string, bool, google.protobuf.Struct, []Value.
• Errors are put under the Error field; error fields (if present) are merged.

[htdvisser]

Not going to do this after all. Instead, we will work with events, and just log to disk/syslog