Update postgres_sequences view

Ref: #500055

What does this MR do and why?

This MR does 2 things:

• Includes all sequences in the system even if not having any dependencies
• Improves our current view to include the following metadata:
  • seq_min
  • seq_max
  • seq_start

[2] pry(main)> Gitlab::Database::PostgresSequence.first
=> #<Gitlab::Database::PostgresSequence:0x000000016f2bb360
 seq_name: "abuse_events_id_seq",
 table_name: "abuse_events",
 col_name: "id",
 seq_max: 9223372036854775807,
 seq_min: 1,
 seq_start: 1>