Migrate time keeping to luatz

**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

Search the timer and scheduler code for uses of the POSIX time library, then review luatz's supported APIs and platforms. Done means fibers' timekeeping uses luatz instead of the current POSIX dependency while preserving timer and scheduler behavior across the stated platforms.

Written by the indexing model from the issue text.