PerWorldInventory Version 2.0.0

Thanks to @Muqsit for their work on this! Here is their pull request.

What's New

1. Drop support for flat-file and migrate to libasynql.
2. Reduce unnecessary database read-writes and inventory updates when teleporting from a to b where a and b are worlds belonging to the same bundle.
3. Add memory database (in-memory SQLite3 database) to store inventories in the memory with a configurable option to whether to keep memory after player quit.
4. [BC-breaking] Change behaviour of Bundled-Worlds.

Bundled-Worlds:
  AllMyCreativeWorldsHere:
    - plot1
    - plot2

The following maps the inventories of worlds plot1 and plot2 to a bundle named AllMyCreativeWorldsHere instead of mapping inventories of worlds AllMyCreativeWorldsHere, plot1 and plot2 to AllMyCreativeWorldsHere world.

Bug fixes

1. Fixes all opened bug issues till issue 29.
2. PerWorldInventory no longer depends on player's gamemode (#20, should probably make this a configurable option though).