Cleanup of new code for allowing "lf_update" to run without extensive locking

- Removed unnecessary variable "direct_voter_update_v" in function "forbid_changes_on_closed_issue_trigger"()
- Added PERFORM "require_transaction_isolation"() in more functions
- Integrated functions "issue_admission", "initiative_admission" and "freeze_after_snapshot" into "check_issue" function
- Removed function "manual_freeze" (replacement: set "issue" timings accordingly, when needed)
- Added/modified some comments