Markus Wanner

Support for Copy on Write semantics (reflink)

3

These two commits add support for Copy on Write semantics to `fs_extra` via the `reflink` crate.

## What Changed? For PGD 5 limitations, clarify the multi-database deprecation. Indicate future plans and a work-around.

## What Changed? Add hints (Markdown comment?) to indicate PGD upgrade consideration that likely is generic and not specific to manual upgrades. NOT TO BE MERGED AS IS. LIKELY JUST...

## What Changed? For command `CLUSTER`, PGD 6 now acquires a global DML lock. Make sure to reflect this in the DDL command handling matrix.