CLEANUP: lua: Remove any ambiguities about lua txn execution context flags

Flags used to set the execution context of a lua txn are used as an enum. It is
not uncommon but there are few flags otherwise. So to remove ambiguities, a
comment and a _NONE value are added to have a clear definition of supported
values.

This patch should fix the issue #1429. No backport needed.
1 file changed