minor cleanup
Provide some tracing to see what's going on.
Ignore more link types
Fix deadlock in sql backend

The connection pool is already thread-safe; it doesn't need
a lock (and will deadlock,du to condition wait)
Work on parallel for-each in scheme
Bug-fix: don't create counters when count is zero
compute-mi: another divide-by-zero bug.
SimpleTrutValue's don't have a count.