Further Load Balancer improvements.

[mashhurs]

Description

Load balancer

Currently we have a minimal promise pattern that guarantees the balance with both cool off and back off periods. Backoff period now covers general broad case. If LS has multiple hosts and it has only single host failure, back off might not be useful. From current point of view we found that it is not an super urgent since we have milliseconds interval sleeps but in the future we may improve this area.

Test cases

With LB implementation, we simply covered the logic unit tests cases but some (load_balancer_spec.rb) of them are tightly coupled or some (in logstash_output_spec.rb) tests can be improved.