schema: rework index sequence API

Rather than passing 'sequence_part' along with 'sequence' on index
create/alter, pass a table with the following fields:

 - id: sequence id or name
 - field: auto increment field id or name or path in case of json index

If id is omitted, the sequence will be auto-generated (equivalent to
'sequence = true'). If field is omitted, the first indexed field is
used. Old format, i.e. passing false/true or sequence name/id instead of
a table is still supported.

Follow-up #4009