Bump Akka to 2.6.20

[PerWiklander]

Since 2.6.20 will be the last Apache 2 licensed Akka version for a while, it sounds like a good idea to get this project synced.

This was mentioned in #131, but I think a separate ticket is better.

[PerWiklander]

I've hade a stab at it.
The most noticeable changes were around Circuit Breakers. I got it to compile, but now I have to have a look at the code provided by this project and try to understand what is changed from upstream earlier and see what more should be updated now.

I had a look at the thesis, but is it possible to give a quick overview of "what goes where" and the reson for reimplementing certain parts?

[andreaTP]

Hi @PerWiklander , I agree a separate issue is good, thanks for taking care of it!

If it were me I would do something along those lines:

• bump the upstream Akka version in sbt
• compile akka-actor module
• compile the akka testkit
• compile and run the tests for akka-actors
• repeat the previous steps for akka-streams and akka-typed

Regarding your questions:

• "what goes where": I think it's better to go case by case, in general, this repo contains just the "diff" to get Akka running on JS VMs
• reimplementing certain parts?: this is easy :-) the parts re-implemented are not compatible with JS and the single threading model

[PerWiklander]

What I was trying to get at:

While patching Akka for JS compatibility, whole files have been added to this repo where a few lines might then be changed for the JS purpose.

If other parts of those files have changed in the new Akka release (new features, bug fixes), we want those changes in this repo as well, not just to pass tests and compilation, right?

[andreaTP]

The short answer is: "possibly yes".

The assumption is:
if the relevant tests are passing everything should be good.

[PerWiklander]

Status update: Got stuck in work stuff. Will try to get this going again next week.

[an-tex]

@PerWiklander #127 this might help as a blueprint