Introduce cursor tracking akin to jdbc input

[jsvd]

With this change it's possible to configure a "tracking_field" whose value is injected into the query using the :last_value keyword.
Example configuration:

input {
  elasticsearch {
    id => tail
    hosts => [ 'http://localhost:9200' ]
    index => 'test-alias'
    query => '{ "query": { "range": {"event.ingested": { "gt": ":last_value", "lte": "now"}}}, "sort": [ { "event.ingested": "asc" } ] }'
    tracking_field => "[event][ingested]"
    slices => 2
    schedule => '*/30 * * * * *'
  }
}
output { null {} }

resolves #184
resolves #93