balancer: SubConn doesn't recover if UpdateAddress() is called with an empty list

[menghanl]

sc.UpdateAddress() closes the addrConn. So if it's called later with an non-empty list, the connection will not recover.

grpc-go/balancer_conn_wrappers.go

Lines 245 to 248 in 36d8757

	if len(addrs) <= 0 {
	acbw.ac.cc.removeAddrConn(acbw.ac, errConnDrain)
	return
	}

[menghanl]

This isn't a big problem for pick-first because zero address is an error:

grpc-go/pickfirst.go

Lines 66 to 69 in 36d8757

	if len(cs.ResolverState.Addresses) == 0 {
	b.ResolverError(errors.New("produced zero addresses"))
	return balancer.ErrBadResolverState
	}

But other balancers (e.g. grpclb) will fail (grpclb fixed in #4879)

[easwars]

Current code puts the subConn in IDLE if it is updated with an empty address list, which seems the right thing to do.

Also, UpdateAddresses is now marked deprecated and will soon be deleted.