Test storage adapters allow process to exit

[lukechilds]

Test storage adapters allow process to exit.

Otherwise the process may hang if the storage adapter keeps a connection open.

We should add something like this in the constructor:

process.on('exit', () => {
    this.opts.store.closeConnection && this.opts.store.closeConnection();
});

Related: https://github.com/lukechilds/keyv-redis/issues/12

[dandv]

This is also a problem with MongoDB. The process never exits because the mongo adapter doesn't close the connection.

Workaround: this.opts.store.db.close();.