Add index on id columns

[MuhammadTahaNaveed]

• Whenever a label will be created, indices on id columns will be created by default. In case of vertex, a unique index on id column will be created, which will also serve as a unique constraint. In case of edge, a non-unique index on start_id and end_id columns will be created.

• This change is expected to improve the performance of queries that involve joins. From some tests, it was observed that the performance of queries improved alot.

• Loader was updated to insert tuples in indices as well. This has caused the loader to slow down a bit, but it was necessary.

• A bug related to command ids in cypher_delete executor was also fixed.