[TestLoop] Make MessageWithCallback accept future as a result.

[robin-near]

Previously, MessageWithCallback<T, R> contains T as well as a callback that is a function that accepts an Result<R, AsyncSendError>. And then, AsyncSender<T, R> is simply Sender<MessageWithCallback<T, R>>.

The problem was that in the actix implementation of Sender<MessageWithCallback<T, R>>, because on the actix side when we call .send(msg) it gives us a future of R, we cannot call the callback right away. The solution was to actix::spawn a future that awaits this future of R and then we call the callback.

This was a bad design, because it is actually the sender (code that calls .send_async(...)) who ends up calling actix::spawn, and that panics if the sender was not on an actix thread.

This PR changes that so that MessageWithCallback<T, R> contains a callback that accepts a future of the result. That way, it becomes the responsibility of whoever awaits on the resulting future from .send_async(...) to drive the result future, and there won't be any problems since we don't spawn anything anymore.

Also correct a use case where code in the test was sending a custom MessageWithCallback. This is not supported; changed it to spawning a future on the testloop instead.

[pugachAG]

LGTM 👍

[codecov]

Codecov Report

Attention: Patch coverage is 85.36585% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.85%. Comparing base (16fd9e8) to head (afa4130).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
core/async/src/actix.rs	71.42%	2 Missing ⚠️
core/async/src/functional.rs	0.00%	2 Missing ⚠️
core/async/src/messaging.rs	87.50%	0 Missing and 1 partial ⚠️
...egration-tests/src/test_loop/utils/transactions.rs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12212      +/-   ##
==========================================
+ Coverage   71.75%   71.85%   +0.09%     
==========================================
  Files         825      827       +2     
  Lines      165822   166639     +817     
  Branches   165822   166639     +817     
==========================================
+ Hits       118993   119732     +739     
- Misses      41634    41691      +57     
- Partials     5195     5216      +21     

Flag	Coverage Δ
backward-compatibility	0.17% <0.00%> (-0.01%)	⬇️
db-migration	0.17% <0.00%> (-0.01%)	⬇️
genesis-check	1.25% <0.00%> (-0.01%)	⬇️
integration-tests	38.79% <41.46%> (-0.14%)	⬇️
linux	71.54% <85.36%> (+0.10%)	⬆️
linux-nightly	71.41% <85.36%> (+0.07%)	⬆️
macos	54.60% <73.07%> (+0.22%)	⬆️
pytests	1.57% <0.00%> (-0.01%)	⬇️
sanity-checks	1.37% <0.00%> (-0.01%)	⬇️
unittests	65.65% <80.55%> (+0.13%)	⬆️
upgradability	0.21% <0.00%> (-0.01%)	⬇️

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