FYI: Very compact "lean" version of DelegatingScheduler

[magicprinc]

This is the continuation of #349

Here:
https://github.com/magicprinc/failsafe/commits/leap_of_faith

Final memory balance:
-1 fat object CompletableFuture
-1 lambda Callable in DelegatingScheduler.schedule
-1 Callable-Runnable wrapper in delayer().schedule (Runnables are wrapped as Callables in FutureTask ctor)

+1 very lean object ScheduledCompletableFuture implements ScheduledFuture, Callable
(not a CompletableFuture anymore)

I am sure this is the final step and one can't optimize this class further.
Not a single unused byte in memory!

[magicprinc]

I did a (not scientific) memory test:

Old scheduled task in memory: 193 bytes
New scheduled task in memory: 136 bytes
70% of original

https://github.com/magicprinc/failsafe/blob/9f23989e3bf77e9b525d0863ba89b89075240355/core/src/test/java/dev/failsafe/internal/util/DelegatingSchedulerTest.java#L404