msgReader implemented in terms of ChunkReader

This should substantially reduce memory allocations and memory copies.

It also means that PostgreSQL messages are always entirely buffered in memory
before processing begins. This simplifies the message processing code.

In particular, Conn.WaitForNotification is dramatically simplified by this
change.

replication_test.go

@@ -1,6 +1,7 @@
 package pgx_test
 
 import (
+	"context"
 	"fmt"
 	"github.com/jackc/pgx"
 	"reflect"
@@ -88,11 +89,10 @@ func TestSimpleReplicationConnection(t *testing.T) {
 	for {
 		var message *pgx.ReplicationMessage
 
-		message, err = replicationConn.WaitForReplicationMessage(time.Duration(1 * time.Second))
-		if err != nil {
-			if err != pgx.ErrNotificationTimeout {
-				t.Fatalf("Replication failed: %v %s", err, reflect.TypeOf(err))
-			}
+		ctx, _ := context.WithTimeout(context.Background(), time.Second)
+		message, err = replicationConn.WaitForReplicationMessage(ctx)
+		if err != nil && err != context.DeadlineExceeded {
+			t.Fatalf("Replication failed: %v %s", err, reflect.TypeOf(err))
 		}
 		if message != nil {
 			if message.WalMessage != nil {