Logical replication woes

Spotted while testing pg_recvlogical:

1. Set up pg_recvlogical to receive:

./pg_recvlogical -S fooslot -d postgres --create
./pg_recvlogical -S fooslot -d postgres --start -f -

2. In another terminal, with psql:

create table foo (id int4);
begin;
insert into foo values (4);
alter table foo alter column id type text;
prepare transaction 'foo';
commit prepared 'foo';
insert into foo values (1);

3. With current HEAD, after commit
bb38fb0d43c8d7ff54072bfd8bd63154e536b384, this produces an assertion
failure:

TRAP: FailedAssertion("!(((xid) != ((TransactionId) 0)))", File:
"reorderbuffer.c", Line: 508)

I believe that's we no longer assign another XID to the transaction that
does the COMMIT PREPARED. Previously, an extra XID, in addition to the
XID of the prepared transaction, was assigned for use in locking the
global transaction entry in shared memory, but that's no longer required.

However, even with that patch reverted, it doesn't work correctly:

ERROR: could not map filenode "base/12142/16390" to relation OID
LOG: starting logical decoding for slot fooslot
DETAIL: streaming transactions committing after 0/16D1670, reading WAL
from 0/16BC470

- Heikki