SAMZA-2353: Support side inputs and standby containers with transactional state

[bkonold]

Feature: Consumption of standby side inputs must honor checkpointed changelog offsets, else customers using standby containers cannot enable transactional state.

Changes: Adds a thread to ContainerStorageManager to continually poll the checkpoint topic, as well as barrier conditions within TaskSideInputHandler to keep consumption of standby side inputs from continuing past the checkpointed offset.


Tests: No tests. TODO.