Add an option to allow using cpimport for LDI and INSERT..SELECT in a transaction

CS has a session variable, columnstore_use_import_for_batchinsert, that has 2 values, ON (default) and OFF. This allows a user to set the option of using cpimport (ON) or the regular DML (OFF) for LDI and INSERT SELECT statements.

If a user performs these queries in a transaction (such as by setting autocommit=0 or using the START TRANSACTION statement), however, CS fallsback to using DML for the operation. This behaviour is because of the current limitation of cpimport to handle rollbacks.

We want to add a third option to columnstore_use_import_for_batchinsert: ALWAYS. This value will always use cpimport for LDI and INSERT SELECT queries, irrespective of whether the query runs in a transaction or not. This gives the user the ability to use a faster import method, with the caveat that if a user issues a rollback of the transaction, it will have no effect as the data would have already been committed to actual database files by cpimport.