config: Improve handling of Deprecated config values
local: Remove tag cache support

- Updates doc references to tag cache
- Removes tag cache from config and marks it deprecated
- Removes tag cache from setup.py
- Removes tag cache from config converter
- Removes tag cache from tests
- Converts local library tests to use JSON.
local: Split out library reading and writting

- Create $XDG_DATA_DIR/mopidy/local in the local extension's validate env.
- Make sure we handle bad data causing ValueError in JSON decoding
- Initializing empty file causes more harm than good as it just leads to a
  ValueError. Switched to doing write_library(json_file, {})
- Helpers have been updated to be library oriented, not track. This paves the
  way for having {tracks: {uri: ...}, artist: {uri: ...}, ...} type
  denormalized data.