Remove SelectValueTo

Benchmarks revealed that it is no longer performant enough to pull
its own wait. Using go_db_bench to copy JSON results to HTTP responses
it was ~20% *slower* for ~4BK responses and less than 10% faster for
+1MB responses.

The the performance problem was in io.CopyN / io.Copy. io.Copy
allocates a 32KB buffer if it doesn't have io.WriterTo or io.ReaderFrom
available. This extra alloc on every request was more expensive than
just reading the result into a string and writing it out to the response
body.

Tests indicated that if MsgReader implemented a custom Copy that used a
shared buffer it might have a few percent performance advantage. But the
additional complexity is not worth the performance gain.

msg_reader.go

@@ -178,20 +178,3 @@ func (r *MsgReader) ReadString(count int32) string {
 
 	return string(b)
 }
-
-func (r *MsgReader) CopyN(w io.Writer, count int32) {
-	if r.err != nil {
-		return
-	}
-
-	r.msgBytesRemaining -= count
-	if r.msgBytesRemaining < 0 {
-		r.Fatal(errors.New("read past end of message"))
-		return
-	}
-
-	_, err := io.CopyN(w, r.reader, int64(count))
-	if err != nil {
-		r.Fatal(err)
-	}
-}