This commit introduces a way to fetch the "serialized voters" for
multiple polls.
* Use a single query to fetch voters for all types of polls
* Refactor to introduce all_serialized_voters
* Cache serialized voters
`track_sql_queries` only returned queries that were executed by
ActiveRecord. All queries executed through DB.exec, DB.query and others
were not returned.
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?).
Since the poll post handler runs very early in the post creation
process, it's possible to run the handler on an obiviously invalid post.
This change ensures the post's `raw` value is present before
proceeding.
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
