Improvements to migrations

[ChrisJollyAU]

• Database model did not set the sort order for the columns in an index
• Try and use base class function for variaous in the JetMigrationsSqlGenerator
  • Our current code for generating the index to did add the sort order for the columns whereas the base class was 99% similar and did add that info
  • Some other functions were exactly the same as base
• Override some functions so that we can add an EndCommand. This finishes the current command sql and starts a new command with a blank string builder. In effect probably does nothing as JetCommand already would split it if all the statements were in one string
• Update the Generate function for the AlterColumnOperation
  • If the computed sql has changed the operation to re-add it (we drop then re-add) now recreates precision,scale,fixedlength etc. Note: as Jet doesn't work on computed sql this is just here for completeness
  • Check against OldColumn to not generate unnecessary ALTER TABLE
  • If the column is being made non-nullable (when originally null), create an UPDATE statement to set the value of that column to the default value for those where the current value is NULL
  • Above changes come from similar improvement on the SQL Server AlterColumnOperation