This commit slignly changes the order of entries in the raft log:
ConfChange now comes at the first place before other Ops, but that's ok.

Do it only when the governor tells to. When `_G.vshard == nil`, it's
more alike `traft::node::global()` which reurns either `Ok` or
`Err(Uninitialized)`.

It produces better error messages.

Before this commit:

```
tarantool> vshard.router.call()
---
- error: '[string "local log = require(''log'')..."]:575: attempt to
    compare number with nil'
...
```

With this commit:

```
- error: 'vshard/router/init.lua:575: attempt to compare number with
    nil'
```