* Update to avoid pseudo-random number

This PR update the usage of rand so that non-global seed is used.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add concurrency-safe random source

See https://stackoverflow.com/questions/48958886/how-to-create-a-thread-safe-rand-sourceSigned-off-by: Yong Tang <yong.tang.github@outlook.com>