Support checksum VFS

[rogerbinns]

https://www.sqlite.org/cksumvfs.html

• [ ] setup fetch needs to retrieve it
• [ ] verify checksum?
• [ ] mechanism to use it
• [ ] doc
• [ ] testing

[rogerbinns]

On further investigation it can't be used. It becomes the default VFS and is ideal where the VFS are static like with C code. It doesn't cope with VFS changing, and makes it backend VFS be whatever was default at time of its loading.

To make it more workable I'd need to do the following code changes to it:

• Supply the name it operates under and the name of the backend VFS
• Audit that it gets dynamically loaded stuff right
• iVersion field needs to be min of what this supports (3) and backend VFS because this will be out of date when iVersion 4 comes out but its own vfs is v3.

Not going to bother.