Specify a log level in tracing instrument macro explicitly.
Additionally reduce the used log level from a default info level to a
debug level (all of those appear to be developer oriented logs, so there
should be no need to include them in release builds).
Reduce memory consumption by sharing the previous dependency graph's
edges with the current graph when it is known to be valid to do so. It
is known to be valid whenever we mark a node green because all of its
dependencies were green. It is *not* known to be valid when we mark a
node green because we re-executed its query and its result was the same
as in the previous compilation session. In that case, the dependency set
might have changed (we don't try to determine whether or not it changed
and whether or not we can share).
