Prefer pg_any_to_server/pg_server_to_any over pg_do_encoding_conversion.

A large majority of the callers of pg_do_encoding_conversion were
specifying the database encoding as either source or target of the
conversion, meaning that we can use the less general functions
pg_any_to_server/pg_server_to_any instead.

The main advantage of using the latter functions is that they can make use
of a cached conversion-function lookup in the common case that the other
encoding is the current client_encoding.  It's notationally cleaner too in
most cases, not least because of the historical artifact that the latter
functions use "char *" rather than "unsigned char *" in their APIs.

Note that pg_any_to_server will apply an encoding verification step in
some cases where pg_do_encoding_conversion would have just done nothing.
This seems to me to be a good idea at most of these call sites, though
it partially negates the performance benefit.

Per discussion of bug #9210.

src/backend/tsearch/ts_locale.c

@@ -209,10 +209,7 @@ t_readline(FILE *fp)
 	(void) pg_verify_mbstr(PG_UTF8, buf, len, false);
 
 	/* And convert */
-	recoded = (char *) pg_do_encoding_conversion((unsigned char *) buf,
-												 len,
-												 PG_UTF8,
-												 GetDatabaseEncoding());
+	recoded = pg_any_to_server(buf, len, PG_UTF8);
 	if (recoded == buf)
 	{
 		/*