For DB connections that want to keep a session alive, there's a new
`DatabaseSession` class that holds a SQLAlchemy session inside and
offers methods similar to the `Database` class.
The `Database` moves to use the `DatabaseSession` to remove duplicated
code.
The database `Client` wasn't working correcly when no session was
passed, as the inserted transactions would still be bound to the newly
created session inside.
Creates a copy of the input transactions to insert on the DB.
`add` method replaced with `insert`.
`insert` and `select` implemented for new database base class.
Database unit test added.
Due to SQLite implementation of the primary key autoinc, the type of the
IDs on the database for SQLite changed to Integer.
https://www.sqlite.org/autoinc.html
