[mem] In memory store for data

[at15]

Related https://github.com/libtsdb/libtsdb-go/issues/1 Closed columnar format in memory https://github.com/xephonhq/xephon-k/issues/49 original tracker https://github.com/xephonhq/xephon-k/issues/3

• [ ] review old in memory store
• don't care too much about read performance when filter
• [ ] try to keep it in sync with libtsdb, though having copy in client side between different protocol is not a big issue ...
• [ ] try with large amount of data, maybe larger than 4GB, to see if it still works
  • https://github.com/allegro/bigcache
• [ ] might use custom map implementation, default map is not that good ...