Improve SKIP LOCKED implementation

[hvlad]

Make SKIP LOCKED works in all transaction isolation levels, including RC NRV
Avoid preliminary lock for UPDATE and DELETE statements

More details at thread "SKIP LOCKED problems" in fb-devel
https://groups.google.com/g/firebird-devel/c/BIbAz_YJMFw

[hvlad]

The fix #7811 introduced bug in DELETE RETURNING handling - calculated fields now returned as NULL's.
Thus some additional work is required.
Also, documentation should be updated.
Re-opened to fix these issues.