Revamp age csv loader (#2044)

[MuhammadTahaNaveed]

• Allow 0 as entry_id
• Use batch inserts to improve performance
  • Changed heap_insert to heap_multi_insert since it is faster than calling heap_insert() in a loop. When multiple tuples can be inserted on a single page, just a single WAL record covering all of them, and only need to lock/unlock the page once.
  • BATCH_SIZE is set to 1000, which is the number of tuples to insert in a single batch. This number was chosen after some experimentation.
  • Change some of the field names to avoid confusion.
• Use sequence for generating ids for edge and vertex
  • Sequence is not used if the id_field_exists is true in load_labels_from_file function, since the entry id is present in the csv.
• Add function to create temporary table for ids, this is only used for loading vertices
  • A temporary table is created and populated with already generated vertex ids when first time load_labels_from_file function is called. A unique index is created on id column to ensure that new ids generated (using entry id from csv) are unique. This table and index will be deleted automatically whenever the session ends.
  • Whenever a row is inserted in labels, the corresponding id is inserted into temp table as well.
• Add functions to create graph and label automatically
  • These functions will check existence of graph and label, and create them if they don't exist.