[YSQL] Feature Support - Transaction Control Commands

[nocaway]

Jira Link: DB-1016
The following transaction control commands are not yet supported.

• PREPARE
• COMMIT PREPARED
• ROLLBACK PREPARED
• DEFERRABLE transaction

As well as savepoint related commands: #9219

[jaki]

DEFERRABLE is covered in issue #2161.

[andrew-s]

I'd like to see this, my particular use case is for automated migrations.

[danports]

Would be nice to have this for pgloader, which uses SAVEPOINT:

Database error 0A000: SAVEPOINT <transaction> not supported yet
>        HINT: See https://github.com/YugaByte/yugabyte-db/issues/1125. Click '+' on the description to raise its priority
>        QUERY: savepoint pgloader;

[ddorian]

SAVEPOINT is also used in Elixir ecto_sql for database migrations. Mentioned in slack.