RFR 8156501: DRBG not synchronized at reseeding

[Wang Weijun]

Hi All

Please review the fix at

   http://cr.openjdk.java.net/~weijun/8156501/webrev.00

Many thanks to Siba for discovering this problem. I have only benchmarked nextBytes() before.

Some clarifications:

1. No need to synchronize configure anymore() because it's always called inside a constructor.

2. synchronized-between-double-check in engineNextBytes() to protect reseedCounter.

Thanks
Max