making tid and HOTness of UPDATE available to logical decoding plugins

Please find attached a patch that makes tuple ids and info about
weather it was plain or HOT update available to logical decoding
callbacks.

Also modified test_decoding to show both tids -
- old tid has format -(pageno, slot)
- new tid has format +(pageno, slot)
if it is a HOT update, it is decoded prefixed with 'HOT '

Sample usage:

hannu=# SELECT pg_create_logical_replication_slot('test_slot', 'test_decoding');
pg_create_logical_replication_slot
------------------------------------
(test_slot,0/1BF1B38)
(1 row)
hannu=# CREATE TABLE nokey(data text);
CREATE TABLE
hannu=# insert into nokey (data) values('a');
INSERT 0 1
hannu=# update nokey set data = 'b';
UPDATE 1
hannu=# delete from nokey ;
DELETE 1
hannu=# SELECT lsn, xid, data FROM
pg_logical_slot_get_changes('test_slot', NULL, NULL);
lsn | xid | data
-----------+-----+------------------------------------------------------------
0/1C20538 | 767 | BEGIN 767
0/1C2B1E8 | 767 | COMMIT 767
0/1C2B220 | 768 | BEGIN 768
0/1C2B220 | 768 | table public.nokey: INSERT:+(0,1) data[text]:'a'
0/1C2B290 | 768 | COMMIT 768
0/1C2B300 | 769 | BEGIN 769
0/1C2B300 | 769 | table public.nokey: HOT UPDATE:-(0,1)+(0,2) data[text]:'b'
0/1C2B378 | 769 | COMMIT 769
0/1C2B3B0 | 770 | BEGIN 770
0/1C2B3B0 | 770 | table public.nokey: DELETE:-(0,2) (no-tuple-data)
0/1C2B418 | 770 | COMMIT 770
(11 rows)

My planned use case is for reliable logical replication of tables
without primary key or other declared IDENTITY (as long as there are
no updates on target, or at leas no non-hot updates)

Sending thgis part as an independent patch as there may be other
interesting use cases as well.

--
Hannu