fixup! wal_decoding: Introduce logical changeset extraction.

Improve comments in decode.c
fixup! wal_decoding: Documentation for replication slots and changeset extractio

Test decoding in existing transactions, test some concurrent DDL.

Re-Add check for transactions with assigned xids when starting
changeset extraction (including when creating a logical replication
slot).
now the decoding_in_xact test is a regression test
new test for the „WAL has been deleted“ problem on big inserts
more testing for concurrent ddl/dml