Minor changes to/housekeeping of FutureImpl. Added internal GuardedBy…

[danieldietrich]

… annotation for documentation purpose. Added @ignored unit tests towards #1530.

[danieldietrich]

Not intended to be pulled yet. Locally the tests still fail, most probably because of the changes to exception handling. But these were made already in 2.0.3!? Why did the tests not fail in the CI tests of 2.0.3? Will update the tests accordingly and investigate further.

[danieldietrich]

Why did the tests not fail in the CI tests of 2.0.3?

Because I changed internals of FutureImpl that depended on the dummy impls of java.util.concurrent.Future for our unit tests.

In particular the underlying java.util.concurrent.Future contains now a null value when completed. Before the value of the computation was stored but never used because we store the value of the computation on our own variable which is guarded by a lock. This is important because we need to synchronize the state of our Future ourselves.

This change is backward compatible because the java.util.concurrent.Future was not accessible from the outside. It is still used to control the asynchronous computation in the same way as before.

[codecov-io]

Current coverage is 96.98% (diff: 92.30%)

Merging #1533 into master will increase coverage by <.01%

@@             master      #1533   diff @@
==========================================
  Files            88         88          
  Lines         10638      10640     +2   
  Methods           0          0          
  Messages          0          0          
  Branches       1814       1814          
==========================================
+ Hits          10316      10319     +3   
+ Misses          179        178     -1   
  Partials        143        143          

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