fixup! wal_decoding: Documentation for replication slots and changeset extractio

Improve handling of transactions from other databases.
fixup! wal_decoding: Introduce logical changeset extraction.

Fix copy&pasto
fixup! wal_decoding: pg_recvlogical: Introduce pg_receivexlog equivalent for logical changes

Fix thinko leading to accidentally closing stdout, but continuing to
write to it...
fixup! wal_decoding: Introduce logical changeset extraction.

immediately start prevent wal file removal when creating a slot
fixup! wal_decoding: Introduce logical changeset extraction.

comments
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
removed expensive test