Do not ignore the very last events

[daladim]

Since (a PR to come, probably #63), we're ignoring the very last ETW events that were still in the buffers when we called CloseTrace

We may want to process them, and drop the memory structures when we're sure every event is processed.
See

/// TODO: it _might_ be possible to know whether we've processed the last buffered event, as
///       ControlTraceW(EVENT_TRACE_CONTROL_QUERY) _might_ tell us if the buffers are empty or not.
///       In case the trace is in ERROR_CTX_CLOSE_PENDING state, we could call this after every
///       callback so that we know when to actually free memory used by the (now useless) callback.
///       Maybe also setting the BufferCallback in EVENT_TRACE_LOGFILEW may help us.