Remove file "post open" VOL callback

[fortnern]

Also add "get_wrap_ctx_pre_open" VOL callback and add "wrap_ctx" parameter to file create and file open VOL callbacks to allow this to work.

The main aim of these changes is to set a valid wrap context before making the file open/create VOL callback. This removes the need for post open since we can do everything inside the main open/create calls. This should also allow more flexibility for VOL connector authors to do things in the open/create calls that were previously precluded due to not having a wrap context.

Passthrough authors need to implement the "get_wrap_ctx_pre_open" callback, but they can ignore the "wrap_ctx" parameter to file create/open unless they want to use the same wrap context for all objects in a file.

[fortnern]

We've decided not to go this route, at least for 1.14. I'll leave this up for a bit longer as a draft incase anyone wants to weigh in on the post open/object wrapping code

[derobins]

Closing this for now