Summary:
`BatcherFlushDone` tries to retry the batch if all failures are retryable. If the session is closed, then we do not need to retry, since no one would be interested in the result.
Updated `CreateBatcher` to ensure a valid session is always passed in.
Jira: DB-16657

Test Plan: TEST_F(PgMiniTest, KillPGInTheMiddleOfBatcherOperation) {

Reviewers: sergei, arybochkin

Reviewed By: sergei

Subscribers: yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D44250