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

[Claes Redestad]

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/

Thanks!

/Claes