These fields are often used when serializing topics which may contain
multiple polls. On average, serializing a poll took 2+N queries where N
is the number of options. This change reduces the number of queries to
3, one for each field (Poll#voters_count, PollOption#voters_count and
Poll#has_voted?).
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
