Added indexes to sql commands used to create session tables

[JohnstonCode]

As per https://github.com/symfony/symfony/blob/4.4/UPGRADE-4.4.md#httpfoundation PdoSessionHandler now precalculates the expiry timestamp in the lifetime column, so the index will speed up garbage collection

[javiereguiluz]

Christopher, thanks for this proposal!

I think it's a good idea because, in a real app, this table will be accessed a lot. However, looking at PdoSession class, all SELECT and UPDATE queries use the id column as the only WHERE clause. Should we create the index on that column instead? I don't know, so I'm just asking.

Sorry ... I misread the description of your PR, so everything I said is wrong. Let's forget it 😊

Before merging I'll wait for others to review this. Thanks!

[Nyholm]

Thank you

[javiereguiluz]

Thank you Christopher.