Fall back to extracting sequence name from column default

Krasimir Angelovrequested to merge
ka/replace-table-sequence-ownership into master

What does this MR do and why?

Update Gitlab::Database::Partitioning::ReplaceTable to try extract sequence name from column's default when pg_get_serial_sequence returns null.

References

Numbered steps to set up and validate the change are strongly suggested.

Example:

  1. Remove ownership for a sequence 
    ALTER SEQUENCE uploads_id_seq OWNED BY NONE;
  2. On master 
    > Gitlab::Database::Partitioning::ReplaceTable.new(ApplicationRecord.connection, 'uploads', 'uploads_new', 'uploads_archived', 'id').sequence
=> nil
  3. On this branch 
    > Gitlab::Database::Partitioning::ReplaceTable.new(ApplicationRecord.connection, 'uploads', 'uploads_new', 'uploads_archived', 'id').sequence
=> "uploads_id_seq"

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