Just user event tracing

[epmikida]

Original issue: https://charm.cs.illinois.edu/redmine/issues/1057

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It would be useful to be able to use projections just for user event tracing. For very finegrained applications like ROSS, tracing every event can be expensive and doesn't necessarily add much. The more interesting part is tracing individual phases with user events, which can only be done when the full trace infrastructure is turned on.