Minor refactoring.
Added "delete-node-index" and "delete-relationship-index" functions.
Added "remove-from-node-index" and "remove-from-relationship-index" functions.
Fixed comment identation.
Replaced "add-to-index with" with "add-to-node-index" and "add-to-relationship-index".
Refactor index handling functions.
Define *db* just once.