RFR 8066777: OptimisticTypesPersistence.java should use Files.readAllBytes instead of getting size and then read

[A. Sundararajan]

Please review http://cr.openjdk.java.net/~sundar/8066777/ for 
https://bugs.openjdk.java.net/browse/JDK-8066777.

This fix was suggested by Paul Sandoz by email.
Thanks,
-Sundar