Add support for sequences on existing column

[chands10]

null=yes should not be needed in this case (see first commit) Adds csc2 and ddl support

Provided that:

• The col the sequence is added on is the leftmost field in some key
  • The key can be added in the same schema change as adding nextsequence
  • Or, an existing valid key can even be removed in the same schema change as adding nextsequence
• It is not a partial key
• Nullsort low is enabled
• The key is ascending
• If the key is empty or the highest value in the key is NULL, start sequence from 0

Also note: in strict mode you can convert a column from int b to longlong b dbstore=nextsequence and this is fine even though the old column is only an int

[chands10]

cdb2test Jun 25 15:22:45 2024 in-comdb2db existing_auto_inc.R20240625.4

[chands10]

THANK YOU MARK!!