Add backup and restore functionality

[hydra-yse]

Closes #5, closes #80, closes #42.

This PR aims at giving the user the ability to backup their pending swaps in order to be able to recover and claim them across instances. Since the swap details cannot be modified across instances, the logic just needs to account for inserting non-existent swaps back to the local database. Also, we should check whether funds have already been claimed/sent, and resolve the swaps accordingly.

TODO:

• Add temp directory to lib tests, avoid reusing same database - fixed in Update library tests with hardcoded data #67
• Resolve pending sends if they are present in an older backup

[ok300]

restore_from_backup shouldn't need to know the schema IMO.

I did some research and based on

• https://docs.rs/rusqlite/latest/rusqlite/backup/index.html
• https://docs.rs/rusqlite/latest/rusqlite/struct.Connection.html#method.restore
• What is the right way to use backup feature? rusqlite/rusqlite#1181

restore could be something like

fn restore_db(backup_path: &str, dest_path: &str) -> Result<()> {
    let src_conn = Connection::open(backup_path)?;
    let dest_conn = Connection::open(dest_path)?;

    let backup = Backup::new(&src_conn, &dest_conn)?;
    backup.run_to_completion(5, std::time::Duration::from_millis(250), None)?;

    Ok(())
}

fn main() -> Result<()> {
    restore_db("backup_database.db", "restored_database.db")?;
    Ok(())
}

What do you think, would that be a good solution?