Optimize readVarString.

Before:

BenchmarkReadVarStr4     1000000              1698 ns/op
BenchmarkReadVarStr10    1000000              1812 ns/op

After:

BenchmarkReadVarStr4     2000000               853 ns/op
BenchmarkReadVarStr10    5000000               712 ns/op

This is part ef the ongoing effort to optimize serialization as noted in
conformal/btcd#27.

common.go

@@ -167,7 +167,7 @@ func readVarString(r io.Reader, pver uint32) (string, error) {
 	}
 
 	buf := make([]byte, count)
-	err = readElement(r, buf)
+	_, err = io.ReadFull(r, buf)
 	if err != nil {
 		return "", err
 	}