What does this PR do?

Closes #262

This PR adds infinite retry functionality, and completely refactors the elasticsearch health monitor. In detail

• If the max_retries is set to -1, this enables "infinite" retry, which just sets the ES MaxRetries to MaxInt.
• Because many common error conditions will now retry instead of trickling down to OnFailure, the monitor instead watches the raw HTTP return codes coming from the elasticsearch transport, and checks the deltas.
• The monitor will mark the ES output as unhealthy if zero successful HTTP codes are reported in a given period, and > 0 error conditions are reported across the reporting period

Why is it important?

We want infinite retry.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md or CHANGELOG-developer.md.

How to test this PR locally

• Pull down and build
• Run either standalone or with agent
• Set max_retries to -1:

outputs:
  default:
    log_level: debug
    type: elasticsearch
    hosts: [https://127.0.0.1:9200]
    username: "elastic"
    password: "changeme"
    bulk_max_size: 4096
    max_retries: -1
    ssl:
      verification_mode: none
    shipper:
      log_level: debug
      enabled: true
      bulk_max_size: 2048

• Ensure that events send properly
• Break Elasticsearch
• Run again, ensure that the shipper reports a degraded state