Support SQL tracing

[mrts]

SQLite supports SQL tracing which is truly useful for debugging prepared statements. Similar functionality may be supported by other backends as well.

Tracing works like this:

static void profile(void *context, const char *sql)
{
    fprintf(stderr, "Query: %s\n", sql);
}

int main()
{
    ...
    sqlite3_trace(db, profile, NULL);
    ...
}

[mrts]

Could use the trace/debug callback as well for warnings when errors occur in destructors, e.g. in finalize_sqlite3() in src/sqlite/SQLiteConnection.cpp.