Before this change, num_hashes meant the number of hashes since
the last ID, minus any hashing done on the event data. It made
no difference for Tick events, but logged Transaction events with
one less hash than actually occurred.
From the perspective of the log, when some data's hash is added,
that data is "discovered" by the historian. Another event
might be a "claim" that some signed data belongs to the owner of a
public key.
