Migrate time keeping to luatz

[rslater-cs]

FEATURE REQUEST

1. We should migrate to using the luatz library for our time keeping. Currently fibers uses the posix time library for the timer and scheduler, which could pose portability issues when releasing fibers to open source or just wanting to use it on different systems. luatz keeps time across multiple platforms by supporting syscall, unix, apple flavoured unix, windows, nginx.

This is a nice-to-have feature for us but a should-have for open source-ing