Add a reference to the keyword for log messages

[Muusssi]

This is mainly relevant for Robot Framework results. The log messages are currently tied to suite and test case but not to a keyword it is related to.

In order to refer to keywords, the log messages would need to point to a fingerprint of a keyword tree which is not available until after the processing of the keyword. This requires a change where log messages are handled in batches and are not archived until after the keyword is processed. This change is also related issue #16 since the pooling results requires similar approach.

[Muusssi]

Essentially this functionality is achieved using the execution paths introduced in branch /feature/execution_path. While the execution paths do not explicitly link the log messages to any keyword or keyword tree as proposed originally, the execution paths define at which point in the execution tree the message was logged. In this approach, the messages are linked to specific executions and their position in that execution.