remove unused `update` DB columns

Instead of saving and relying on them,
they are always calculated when needed.
StdlibPending::GetEntries() - remove check for published release

This must be and is ensured by the caller.
remove obsolete and useless API
Add Stdlib::writeEntry()
Add Stdlib::GetItemsUnpublished()

Move the logic for merging base release and
pending changes from Stdlib::GetItems() to a
new separate method Stdlib::GetItemsUnpublished()
add method for previous release to StdlibRelease class
fix StdlibPending::DeleteEntry() column name
add a force_unpublished parameter to StdlibRelease::update()
one more SQL fix to StdlibPending::DeleteEntry()
add new StdlibRelease::publish() method