Remove smart pointers from SQLite veneer

Sqlite now extends tensorflow::core::RefCounted which is a better practice for
code in the TensorFlow codebase.

A few other trivial changes were snuck in. There's now a db->changes() method.
Error messages will also display the SQLite extended result code, which can be
looked up by hand with some difficulty, just in case the error message string
doesn't reflect the whole nuance of something like an i/o error.

PiperOrigin-RevId: 181410358