Update in_tail to use storage plugins instead of pos_file

`in_tail` is using a file to record the positions of files, to restart reading from there after Fluentd process restarts.
But pos_file is formatted in custom binary, and hard to read/edit. And it requires any fixed path to be configured.

Storage plugins (especially storage_local) can store any data on paths configured via `root_dir`. It's very easy to manage, and make configuration files smaller, by reducing each paths (including pos_file) from plugin configuration sections.
`in_tail` should be updated to use storage plugins for positions.

Points to be considered:
* How to convert pos_file to storage plugin data? online update? any converter scripts?
* How to manage/clean-up old data? #1126

Start by reading the in_tail implementation and the storage plugin, especially storage_local, to understand how positions are currently written to pos_file and how root_dir-backed data is managed. Investigate the migration questions, including converting existing pos_file data and cleaning up old data related to #1126. Done means positions use storage plugins with a clear upgrade and cleanup path.

Written by the indexing model from the issue text.