Allow logs to be appended during run

[rowleya]

When using SpiNNaker, the SpiNNaker client currently does "live uploads" of logs. I believe that this is causing problems when the user outputs a lot of logging data, as it is currently done by uploading the whole log again each time it is updated. This could be better supported if the service end had a log "append" option, which would simply add the text sent to the log rather than overwriting it.