add random pool to reduce concurrency contention on genSeededGUID()

[lzuwei]

• genSeededGUID() has a potential to content on a single mutex lock with high parallelism as observed in our production system at Grab
• mitigate this issue using a pool of random generators wrapped with a lock to reduce contention

[jmacd]

The interface should have two methods, func Int63() int64 and func TwoInt63() (int64, int64) or something like that.

[lzuwei]

@jmacd @tedsuo, I have shifted the implementation details into the suggested packages. Please review, thank you!