mpdstats: do not call item.write

The attributes used by this plugin are not 'writable'[^1] anyway so this
leads to needless file access. Also we do not need to check for
`item._lib` as this will always be populated by the call to
`lib.items(...)`.

[^1]: As per `ITEM_KEYS_WRITABLE` in `beets/library.py`
mpdstats: refactor plugin
mpdstats: reload item before calculations

As mpdstats is a long running process the value of one of the attributes
may have changed since the song started to play (f.ex. through a manual
update using `modify`).
Merge pull request #472 from kljohann/mpdstats_refactor

Refactor mpdstats plugin