box: yield after initial box_cfg() is finished

box.cfg() works in two stages, when called first time - boot the
instance using box_cfg() C++ function, and then configure it.
During booting all non-dynamic parameters are read. Dynamic are
configured mostly afterwards.

Normally there should be a yield between box_cfg() C++ call and
dynamic parameters configuration. It is used by box.ctl.wait_ro()
and box.ctl.wait_rw() Lua calls to catch the instance in read-only
state always before read-write state.

In theory a user should be able to call box.ctl.wait_ro() and
box.ctl.wait_rw() in one fiber, box.cfg() in another, and these
waits would be unblocked one after another.

It works fine now, but only because of, surprisingly, the feedback
daemon. The daemon creates a yield after C++ box_cfg() is
finished, but dynamic parameters are still being applied in
load_cfg.lua. That gives time to catch box.ctl.wait_ro() event.

The thing is that dynamic parameters configuration includes the
daemon's options too. When 'feedback_enable' option is installed
to true, the daemon is started using fiber.create(). That creates
a yield, and gives time to box.ctl.wait_ro() fibers to handle the
event.

When the daemon is disabled or removed, like it is going to happen
in #3308, this trick does not work, and box.ctl.wait_ro() started
before box.cfg() is never triggered.

It could be tested on app-tap/cfg.test.lua with

    box.cfg{}

changed to

    box.cfg{feedback_enabled = false}

Then the test would hang. A test is not patched here, because the
feedback is going to be optionally removed in a next commit, and
the test would become flaky depending on build options.

Needed for #3308