Fix ReadFile handler to consider the value stored in sincedb on plugin restart

Release notes

Fixes read mode when sincedb already stores a reference for the file not completely consumed.

What does this PR do?

Update the file pointer of a read mode file to the max between the read bytes or the sincedb reference for the same file. This solves a problem, that when a pipeline is restarted, it's able to recover from the last known reference, without restarting from the beginning, and reprocessing already processed lines.

Why is it important/What is the impact to the user?

When a pipeline with file input in read mode is restarted, this let the plugin to recover from where it left if that information is present in the sincedb store.

Checklist

[x] My code follows the style guidelines of this project
[x] 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 (and/or docker env variables)~~
[x] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

[x] verify with the steps used in the bug report #240. I used the following test file: sample_fixture.csv.txt

Pipeline definition:

- pipeline.id: SDH_650
  pipeline.workers: 1
  pipeline.batch.size: 5
  config.string: |
    input {
        file {
            path => "/home/andrea/workspace/logstash_configs/file_input_sdh650/sample_fixture.csv"
            sincedb_path => "/home/andrea/workspace/logstash_configs/file_input_sdh650/sincedb"
            mode  => "read"
            start_position => "beginning"
        }
    }

    filter {
        csv {
            separator => ","
            columns => ["id", "host", "fqdn", "IP", "mac", "role", "type", "make", "model", "oid", "fid", "time"]
            remove_field => ["path", "host", "message", "@version" ]   
        }
        sleep {
            time => 1
            every => 10
        }
    }

    output {
        elasticsearch { 
            index => "650" 
            hosts => "http://localhost:9200"
            user => "elastic"
            password => "changeme"
        }
        stdout { codec => dots }
    }

Some curls to configure the ES output index and an aggregation query to verify:

PUT /650
{
  "mappings": {
    "properties": {
      "id":    { "type": "keyword" },  
      "host":  { "type": "text"  }, 
      "fqdn":   { "type": "text"  },
      "IP":   { "type": "text"  },
      "mac":   { "type": "text"  },
      "role":   { "type": "keyword"  },
      "type":   { "type": "keyword"  },
      "make":   { "type": "text"  },
      "model":   { "type": "text"  },
      "oid":   { "type": "text"  },
      "fid":   { "type": "text"  },
      "time":   { "type": "text"  }
    }
  }
}
DELETE 650

GET 650/_search
{
  "aggs": {
    "types": {
      "terms": { "field": "type" }
    }
  }
}

The expectation is to have 2 buckets, equally sized. Without the fix a bucket contains more documents, which means some rows was reprocessed on a pipeline reload.

logstash-plugins / logstash-input-file