lua: fix running init lua script

When tarantool launched with -e flag and in
script after there is an error, program hangs.
This happens because shed fiber launches separate
fiber for init user script and starts auxiliary
event loop. It's supposed that fiber will stop
this loop, but in case of error in script, fiber
tries to stop a loop when the last one isn't
started yet.

Added a flag, which will watch is loop started and
when fiber tries to call `ev_break()` we can be sure
that loop is running already.

Fixes #4983