Use transactions in case of InnoDB

[mestaritonttu]

Importing to InnoDB databases is really slow, if transactions are not used.

Here you can see, how transaction support is checked and transactions used in PW core: https://github.com/processwire/processwire/blob/dev/wire/core/WireDatabasePDO.php#L311

[flydev-fr]

Thanks @mestaritonttu I will look into it. What is needed here is to rewrite the installer.