[CT-1191] Memory growth: stop saving Exceptions in logging events

[gshank]

Memory in a long-running server was continually growing because when an error occurred the node was saved in an Exception which was saved in a logging event which was stored in the logging event history. The references to the nodes meant that old manifests could not be garbage collected.

We should change Exceptions in logging events to strings, and add 'event_buffer_size' to UserConfig.