Add filtering support for “online” migrations (logical decoding)

[dimitri]

The wal2json plugin allows for filtering but pgcopydb does not fill-in that setting entry with the list of filtered-out tables from its own setup at the moment. The test_decoding plugin lacks the feature entirely, so in that case we would need to filter-out events on the receiving end.

It might be worth filtering on the receiving end to get started with, so that we share a single implementation for both plugins.

Fixes #238.