Some refactoring of logical/worker.c

Some refactoring of logical/worker.c

This moves the main operations of apply_handle_{insert|update|delete},
that of inserting, updating, deleting a tuple into/from a given
relation, into corresponding
apply_handle_{insert|update|delete}_internal functions.  This allows
performing those operations on relations that are not directly the
targets of replication, which is something a later patch will use for
targeting partitioned tables.

Author: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Rafia Sabih <rafia.pghackers@gmail.com>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+HiwqH=Y85vRK3mOdjEkqFK+E=ST=eQiHdpj43L=_eJMOOznQ@mail.gmail.com