Loggers pointing to the same file step on each other

[Nil Geisweiller]

To reproduce the error comment out the following line

https://github.com/opencog/cogutil/blob/master/tests/util/LoggerUTest.cxxtest#L263

and run testLoggerInteraction(). The message of the first logger
will be cut before the end by the message of the second logger.

Till this get fixed the work around is to flush the first logger
before calling the second logger (as the test indicates).

[Nil Geisweiller]

Note that this issue is concerned about the garbage produced by messages stepping on each other, not about the loose ordering of the messages (which is also annoying but is another problem).