Rename libpq trace internal functions

libpq's pqTraceOutputMessage() used to look like this:

     case 'Z':               /* Ready For Query */
         pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
         break;

Commit f4b54e1ed98 introduced macros for protocol characters, so now
it looks like this:

     case PqMsg_ReadyForQuery:
         pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
         break;

But this introduced a disconnect between the symbol in the switch case
and the function name to be called, so this made the manageability of
this file a bit worse.

This patch changes the function names to match, so now it looks like
this:

     case PqMsg_ReadyForQuery:
         pqTraceOutput_ReadyForQuery(conn->Pfdebug, message, &logCursor);
         break;

(This also improves the readability of the file in general, since some
function names like "pqTraceOutputt" were a little hard to read
accurately.)

Some protocol characters have different meanings to and from the
server.  The old code structure had a common function for both, for
example, pqTraceOutputD().  The new structure splits this up into
separate ones to match the protocol message name, like
pqTraceOutput_Describe() and pqTraceOutput_DataRow().