add separate timestamp for factory-generated events

[jclulow]

The time_remote field on the job event record was added to store the local time provided by the agent when it generated the record. This is distinct from the original time field, which reflects the time at which the buildomat core server received the event from the agent.

Once factories were added, new event record types were added that are not generated on the agent but rather within the factory; e.g., information about the IPMI state of the machine, or output from the machine's serial console. The time in this case comes from the factory, and is thus also distinct from the time on the agent and the time on the core server.

We should add an optional time_factory column to store these timestamps for factory-provided events, and stop using time_remote in that case, leaving time_remote to the agent itself.