* feat(emqx_conf): move conf manager for emqx_machine to emqx_conf
* chore(emqx_conf): change emqx:get_config/2 to emqx_conf:get/2
* fix: common test failed
* fix: badmatch by typo wrong key
* fix(emqx_conf): get the wrong core nodes
* fix(emqx_conf): get core node's tnx_id not latest tnx_id
* fix: add ro_transation when copy conf file
* fix: delete debug info
* fix: change ekka_rlog to mria_rlog
* fix: remove cluster_rpc from emqx_machine.
* fix: don't call ekka:start/0 explicitly
* fix: ekka should be start in emqx_machine
Move the emqx_config:update,remove,reset APIs to emqx, to remove
the circular dependency between the modules emqx_config and
emqx_config_handler.
After this change the dependency among these modules will be:
```
emqx ---> emqx_config
| ^
| |
+ ---> emqx_conifg_handler
```