config: automatically set missing names

For now it's impossible to use config module in order to recover
from snaps, which don't have names set in them. Calling box.cfg
with names on recovery fails with MISMATCH error, which is caused
by difficult implementation of setting names on first box.cfg,
as names can be set only on rw instance.

This commit doesn't call box.cfg with names, if these names are
missing from the snapshot file. Instead it creates a fiber, which
will set names, when it's possible to do so. Only master sets the
names for the whole cluster.

Closes #8978

NO_DOC=tarantool/doc#3661
NO_CHANGELOG=following commits