Better support for large append-only tables by making the snapshotting process restartable

Currently, the snapshotting process is non-interruptible to ensure that snapshotting results in creating a consistent dump of the existing data in the tables before the CDC phase begins.

This is appropriate in the general case, but for very large log-like tables, which are append-only, snapshotting can take a long time (several days in our case for some larger tables), and if this phase is interrupted, it has to be restarted from scratch.

I propose to introduce the option to override the select statement issued during snapshotting. This way it is possible to resume a snapshot that has progressed to a specific point (e.g. by adding a custom WHERE clause with a condition on the record ID).

I have implemented a patch for this feature, for which I'd like to create a pull request.