In SQLite bindings are not cleared by sqlite3_reset() calls, so
skipping a sqlite3_bind call to create a NULL value doesn't work,
instead the previous value will be written.
To fix this, I clear all bindings in SqlQuery::reset and make sure
to explicitly bind NULL when desired in SqlQuery::bind.
To make sure there's no confusion about SqlQuery::reset and
sqlite3_reset, I rename our method to reset_and_clear_bindings().
