SafeSubscriber: move the upstream subscription to a regular field

[jponge]

This removes the AtomicReference / volatile semantics out of the way on the hot path,
at the cost of an additional AtomicBoolean to ensure correctness on multiple onSubscribe
signals.

[codecov]

Codecov Report

Merging #371 (bcd9dcb) into master (1da1b06) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #371      +/-   ##
============================================
+ Coverage     89.04%   89.32%   +0.27%     
- Complexity     2709     2718       +9     
============================================
  Files           361      361              
  Lines         10474    10486      +12     
  Branches       1302     1304       +2     
============================================
+ Hits           9327     9367      +40     
+ Misses          598      584      -14     
+ Partials        549      535      -14     

Impacted Files	Coverage Δ	Complexity Δ
...o/smallrye/mutiny/subscription/SafeSubscriber.java	97.97% <100.00%> (+0.02%)	20.00 <6.00> (ø)
...java/io/smallrye/mutiny/helpers/Subscriptions.java	79.55% <0.00%> (-1.66%)	48.00% <0.00%> (-1.00%)
...a/io/smallrye/mutiny/converters/multi/ToMaybe.java	100.00% <0.00%> (ø)	4.00% <0.00%> (ø%)
...rye/mutiny/operators/multi/MultiRepeatUntilOp.java	100.00% <0.00%> (ø)	5.00% <0.00%> (ø%)
...e/mutiny/streams/stages/FindFirstStageFactory.java	100.00% <0.00%> (ø)	5.00% <0.00%> (ø%)
...mutiny/operators/multi/builders/ResourceMulti.java	87.00% <0.00%> (+0.13%)	5.00% <0.00%> (ø%)
...mallrye/mutiny/operators/UniOnTerminationCall.java	93.54% <0.00%> (+0.21%)	2.00% <0.00%> (ø%)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	86.64% <0.00%> (+0.62%)	3.00% <0.00%> (ø%)
...ye/mutiny/operators/multi/MultiRepeatWhilstOp.java	83.33% <0.00%> (+0.72%)	2.00% <0.00%> (ø%)
...lrye/mutiny/subscription/SerializedSubscriber.java	83.01% <0.00%> (+0.94%)	26.00% <0.00%> (+1.00%)
... and 10 more