Skip logical slot processing in PostgreSQL polling mode

[vvsos1]

Summary

• advance the PostgreSQL polling checkpoint after each successful forward pass
• skip WAL and logical replication-slot processing in polling mode
• add regression coverage

Problem

PGSync documents --polling as operating
instead of logical replication
and requiring
only database read access.

However, Sync.pull(polling=True) still calls logical-slot operations. If
the slot is missing or the user lacks replication privileges, the checkpoint
is not updated, causing the same rows—or the full table—to be scanned again
on every cycle.

Solution

After a successful forward pass, PostgreSQL polling mode now stores the
captured snapshot_xmin checkpoint and returns before accessing the WAL or
logical replication slot.

MySQL/MariaDB and PostgreSQL replication-based modes are unchanged.

Tests

• pytest tests/test_sync_polling.py -vv
• ./scripts/lint.sh

[toluaina]

Great find. and thanks for adding test