Add dev-only query logging support

This PR adds query logging support as an optional feature.
It is only allowed during development/debug builds, and will abort when
used during a `--release` build.

For this feature to be fully activated you also need to se an
environment variable `QUERY_LOGGER=1` to activate the debug log-level
for this crate, else there will be no output.

The reason for this PR is that sometimes it is useful to be able to see
the generated queries, like when debugging an issue, or trying to
optimize a query. Currently i always added this code when needed, but
having this a part of the code could benifit other developers too who
maybe need this.