Optimize transaction lookups.

This commit modifies the transaction lookup code to use a set instead of a
slice (list).  This allows the lookup to automatically prevent duplicate
requests to the database.

Previously, the code simply added every referenced transaction to a list
without checking for duplicates, which led to multiple requests against
the database for the same transaction.  It also meant the request list
could grow quite large with all of the duplicates using far more memory
than required.

While the end result was accurate, operating that way is not as efficient
as only requesting unique transactions.