Depending on sqlx makes users pull SQL related code that is usually not
needed. Tools like dep make it easier by explicitly listing relevant
subpackages but the code needs to be downloaded anyway.
Releasing query objects in Get/Select could easily lead to
double-releases, which can cause dangerous and tricky data races.
Remove the query field of Iterx and its usages (all release-related).
This is a breaking API change, because it removes the exported method
ReleaseQuery.
Update documenting examples to demonstrate the deferred query release
pattern clients can use to manage query release.
