Bug: Concurrency is broken in Schedule

[kyranet]

Describe the issue

If you have 10 tasks and they all end at nearly the same time, chances are 100% that they all will read the state before being updated by the other writes, meaning that the last to write to DB will restore the other 9. Thanks @gc for noticing this bug.

Code or steps to reproduce

• Create a synchronous task.
• Create 10 scheduled tasks for it.
• Wait until the 10th finished.
• You'll have the internal cache empty, but the DB will report 9 objects in schedule.

Expected and actual behavior

It should do either sequentially or in a burst (using @klasa/request-handler to do an individual update then one update containing 9 changes).

Further details

• discord.js version: fe7df708e44e0280dfaf0f8e457b154781bb5140
• node.js version: 12.15.0
• Klasa version: 0.5.0-dev
• I have modified core files.
• I have tested the issue on latest master. Commit hash: 42fdd6d4b8ec1596e26f21e03a415de179b816ba