structured logging: add source location discovery

[minherz]

Is your feature request related to a problem? Please describe.
If not defined, the sourceLocation field in LogEntry should be auto-populated when writing log entries to backend

Describe the solution you'd like
Source location info should be populated implicitly following the format

[minherz]

Related issues #685, #686, #687, #688, #689, #690, #691

[minherz]

Source location auto-population should discover the source location for the caller of the Logging.write() interface by default.
In a case the caller does not call the interface directly (e.g. when it is happened from the logback appender) there should be an option to discover the caller based on hints such as number of calls along the callstack or an entry point which is expected to be called.

[minherz]

The method to get the source location info in Java 8 is to use the stack trace:

StackTraceElement[] stackTrace = (new Exception()).getStackTrace();