Enable write-ahead logging and memory mapped IO

[samuel-w]

Prior pull request #4612, I broke it with a force push.

I set locking_mode to exclusive for highest compatibility with WAL, and got the mmap number from the SQLite documentation.

[gpatel-fr]

Hello

thanks for the PR, however I don't see why it's necessary. With #4907 any Sqlite option can be set.
In my case I set it like that:

CUSTOMSQLITEOPTIONS_DUPLICATI=cache_size=-300000;temp_store=2;journal_mode=WAL;synchronous=Normal

but you can use any option you want.

This has IMO 2 advantages over your PR:

• first it does not impose any particular option to the user who has the possibility to set whatever is felt better so it's better for experts
• Second, enabling the WAL in all cases change the way of copying the database, something that can and do confuse ordinary users so it's better for beginners who could copy databases without much caring if they are opened or not, and what is less confusing for beginners lead to less anguished posts on the forum.

I agree that using WAL is better for insert performance, however Duplicati performance depends on many parameters and enabling WAL can lead to no significant enhancement if other effects are dominant, see my post here:

https://forum.duplicati.com/t/backup-upload-speed/16684/1