And move emqx app back to root level
After realising the challenges of developing plugins in standalone
rebar projects. it is perhaps more smooth to keep using emqx as
a rebar dependency
* Add more test cases for emqx_stats and emqx_os_mon
Fix test case error for emqx_ws_connection
* Add more test cases for emqx_sys_mon
* Update erlang otp to 22.1 for travis ci
* Delete readable=false for make ct
* Add unset_all_env for emqx_zone and update test cases
Before this change, the stats callback provided by emqx_broker_helper
was an anonymous function with module local context.
This commit changes it to a full fun M:F/A style callback for:
1. More robust to hot beam reload
2. Faster/smaller variable to construct
3. Easier test
Zaiming Shi