feat: Add onquery callback.

[stephenh]

Based on @porsager 's query-stats branch with @abustany 's fixes (to manage the 2nd/3rd onquery instances on the query-level instead of global handler level iirc).

Disclaimers that:

1. I did not add tests, and
2. Currently the onquery accepts the Query instance as-is, which can be difficult for APM tools to look at "just give me the SQL string + params", so it might be worth passing in an API that is higher-level/easier-to-consume than the raw Query

Fixes #461.