Output call durations in locale-independent format

[nathklei]

Durations are formatted with decimal separator depending on locale. To avoid confusion be more portable for consumption ideally we'd output with a statically defined locale.

https://github.com/palantir/tritium/blob/develop/tritium-slf4j/src/main/java/com/palantir/tritium/event/log/LoggingInvocationEventHandler.java#L208