Improve microbench.js

Improve microbench.js

- ensure handler behavior does not depend on n argument
- load and save reference file in node.js
- add -s filename option to name the output reference file
- add targets in Makefile for tests and bencharks using node.js
- fix incorrect timings when not using high resolution timer
- use performance timer in node.js
- output performance factor instead of percentage
- use smaller threshold by default
- add benchmarks for:
    date_parse(), prop_update(), prop_clone(), array_slice()
    global_func_call(),