box: rename replica register and on_join funcs

box_on_join() was called not only on IPROTO_JOIN but also on
IPROTO_REGISTER. The name was a bit misleading. It is now called
box_register_replica().

The old box_register_replica() is renamed to
box_insert_replica_record(). It says "insert record", because this
is what it does - inserts a new tuple into _cluster space.

It also skips the check whether the instance is read-only. It
allows to use the function on the bootstrap master so as it could
register itself.

Needed for #5029

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=already covered