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.

bench_test.go

@@ -2,7 +2,6 @@ package pgx_test
 
 import (
 	"github.com/jackc/pgx"
-	"io/ioutil"
 	"math/rand"
 	"testing"
 )
@@ -48,23 +47,6 @@ func BenchmarkSelectValuePreparedNarrow(b *testing.B) {
 	}
 }
 
-func BenchmarkSelectValueToPreparedNarrow(b *testing.B) {
-	conn := mustConnect(b, *defaultConnConfig)
-	defer closeConn(b, conn)
-	createNarrowTestData(b, conn)
-
-	// Get random ids outside of timing
-	ids := make([]int32, b.N)
-	for i := 0; i < b.N; i++ {
-		ids[i] = 1 + rand.Int31n(9999)
-	}
-
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		mustSelectValueTo(b, conn, ioutil.Discard, "getMultipleNarrowByIdAsJSON", ids[i], ids[i]+10)
-	}
-}
-
 func BenchmarkConnPool(b *testing.B) {
 	config := pgx.ConnPoolConfig{ConnConfig: *defaultConnConfig, MaxConnections: 5}
 	pool, err := pgx.NewConnPool(config)