Fix an issue with notifying about changes in Ember 3.13 and upgrade the addon

[tniezurawski]

Fixes: https://github.com/yapplabs/ember-buffered-proxy/issues/48

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I know PR is huge. In fact, the change that fixes the issue with notifying about changes is a one-liner. But I really wanted to refresh the addon as this is something we use in our project a lot.

I added acceptance tests to this project because I think it relies a lot on Ember internals. It's cool that you have unit tests but it's not enough because the whole Ember is just a black box there. We need to be sure that changes are propagated on views when we change buffered properties and when we apply the changes to the content.

It's probably easier to review this change commit by commit but here are the milestones:

• Adding acceptance tests - https://github.com/yapplabs/ember-buffered-proxy/commit/447e08845e5074f031224001c00ef7173de1c809
• Bump Ember from 2.15 to 3.14
  • I went from LTS to LTS version and run codemods
• The fix for notifying about changes - https://github.com/yapplabs/ember-buffered-proxy/commit/46d00d71cc85726025e1c385892d59078640914c
• Drop the support for Ember < 3.8 - that version is not even supported by ember anymore but please see the commit message to get more context - https://github.com/yapplabs/ember-buffered-proxy/commit/bcd460a65f50e9946aec92735061db8eb46da2f9

[lukemelia]

Nice work, @tniezurawski! I sent you a collaborator invite. Welcome to the team. :-)

[tniezurawski]

Thanks @lukemelia! :)