logging: set keep_last_written_value to false

the keep_last_written_value option makes OutputPort::write not thread-safe, which might cause memory corruptions

Signed-off-by: Ruben Smits <ruben.smits@intermodalics.eu>
Merge pull request #15 from orocos-toolchain/make-logging-threadsafe

logging: set keep_last_written_value to false