Enable execute() to run prepared_statements for all connectors

[margaretselzer]

I am absolutely not sure about what applies to PostgreSQL and MySQL in terms of limitation to one query at a time or return value, but I hope that this is in any case a good baseline.

I have taken the liberty to modify the return type of execute() for MySql from void to size_t so that it is in sync with other connectors.

[rbock]

Thanks!

FYI: It might take a few days for me to take a closer look. I am a bit busy with something else right now :-)

Feel free to ping me if it feels like it takes too long.

[margaretselzer]

No worries. Do it when you can.

The only thing depending on this for me right now is query() (and some documentation updates). But lacking that does not block anything for me. Just doin' it for the thrill of a bit of intellectual satisfaction. ;-)