[10] RFR: 8184603: Create ObjectStreamField signature lazily when possible

[Chris Hegarty]

> On 14 Jul 2017, at 13:19, Claes Redestad <claes.redestad at oracle.com> wrote:
> 
> Hi,
> 
> as a small startup optimization, we could make some setup in ObjectStreamField
> lazier by calculating the signature field at first use.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8184603
> Webrev: http://cr.openjdk.java.net/~redestad/8184603/jdk.00/

I think this is ok.  Should toString also use getSignature?

For extra points remove the superfluous null assignment of
`signature` and `field` ( as well as `offset` ).

-Chris.