Add new eventlog EVENT_WALL_CLOCK_TIME for time matching
Eventlog timestamps are elapsed times (in nanoseconds) relative to the process start. To be able to merge eventlogs from multiple processes we need to be able to align their timelines. If they share a clock domain (or a user judges that their clocks are sufficiently closely synchronised) then it is sufficient to know how the eventlog timestamps match up with the clock. The EVENT_WALL_CLOCK_TIME contains the clock time with (up to) nanosecond precision. It is otherwise an ordinary event and so contains the usual timestamp for the same moment in time. It therefore enables us to match up all the eventlog timestamps with clock time.