netbox: fix leak of connection with set reconnect_after option

If a connection has reconnect_after > 0, then it is never deleted
until it is explicitly closed or reconnect_after is reset. It is
because worker fiber of such connection holds all references
during yield.

Fix it by do not waiting for a next reconnection inside a state
machine - protocol_sm() function must not be infinite in a case
of error.

Closes #3164