pgsql: Use Enum for top level logical replication message types.

Use Enum for top level logical replication message types.

Logical replication protocol uses a single byte character to identify a
message type in logical replication protocol. The code uses string
literals for the same. Use Enum so that

1. All the string literals used can be found at a single place. This
makes it easy to add more types without the risk of conflicts.

2. It's easy to locate the code handling a given message type.

3. When used with switch statements, it is easy to identify the missing
cases using -Wswitch.

Author: Ashutosh Bapat
Reviewed-by: Kyotaro Horiguchi, Andres Freund, Peter Smith and Amit Kapila
Discussion: https://postgr.es/m/CAExHW5uPzQ7L0oAd_ENyvaiYMOPgkrAoJpE+ZY5-obdcVT6NPg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/644f0d7cc9c2cb270746f2024c706554e0fbec82

Modified Files
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src/backend/replication/logical/proto.c | 26 +++++-----
src/backend/replication/logical/worker.c | 87 ++++++++++++++++----------------
src/include/replication/logicalproto.h | 27 ++++++++++
3 files changed, 83 insertions(+), 57 deletions(-)