Enumize logical replication message actions

Hi All,
Logical replication protocol uses single byte character to identify
different chunks of logical repliation messages. The code uses
character literals for the same. These literals are used as bare
constants in code as well. That's true for almost all the code that
deals with wire protocol. With that it becomes difficult to identify
the code which deals with a particular message. For example code that
deals with message type 'B'. In various protocol 'B' has different
meaning and it gets difficult and time consuming to differentiate one
usage from other and find all places which deal with one usage. Here's
a patch simplifying that for top level logical replication messages.

I think I have covered the places that need change. But I might have
missed something, given that these literals are used at several other
places (a problem this patch tries to fix :)).

Initially I had used #define for the same, but Peter E suggested using
Enums so that switch cases can detect any remaining items along with
stronger type checks.

Pavan offleast suggested to create a wrapper
pg_send_logical_rep_message() on top of pg_sendbyte(), similarly for
pg_getmsgbyte(). I wanted to see if this change is acceptable. If so,
I will change that as well. Comments/suggestions welcome.

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Best Wishes,
Ashutosh Bapat