Add missing break out seqscan loop in logical replication

When replica identity is FULL (an admittedly unusual case), the loop
that searches for tuples in execReplication.c didn't stop scanning the
table when once a matching tuple was found.  Add the missing 'break'.

Note slight behavior change: we now return the first matching tuple
rather than the last one.  They are supposed to be indistinguishable
anyway, so this shouldn't matter.

Author: Konstantin Knizhnik
Discussion: https://postgr.es/m/379743f6-ae91-b866-f7a2-5624e6d2b0a4@postgrespro.ru
